WORKS  OF  PROF.  F.  P.  SPALDING 

PUBLISHED    BY 

JOHN  WILEY  &  SONS 


A  Text-book  on  Roads  and  Pavements. 

The  aim  of  this  work  is  to  give  a  brief  discus- 
sion, from  an  engineering  standpoint,  of  the 
principles  involved  in  highway  work,  and  to 
outline  the  more  important  systems  of  con- 
struction, with  a  view  to  forming  a  text  which 
may  serve  as  a  basis  for  a  systematic  study  of 
the  subject.  Fourth  Edition  Revised  and  partly 
Rewritten.  12mo,  xii  +  408  pages,  51  figures. 
Cloth,  $2.00  net. 

Hydraulic  Cement:   Its  Properties,  Testing,  and 
Use. 

This  work  embodies  the  results  of  a  careful 
study  of  the  nature  and  properties  of  hydraulic 
cement,  and  the  various  methods  which  have 
been  proposed,  or  are  in  use,  for  testing  cement. 
Second  Edition,  Rewritten.  12mo,  x  +  300 
pages,  34  figures.  Cloth,  $2.00. 


A  TEXT-BOOK 

ON 

ROADS  AND  PAVEMENTS 


BY 

FREDERICK    P.  SPALDING 

*^ 

PROFESSOR    OF    CIVIL    ENGINEERING,    UNIVERSITY   OF    MISSOURI,    MEM1 
AMERICAN    SOCIETY    OF    CIVIL    ENGINEERS 


FOURTH   EDITION,  REVISED   AND   ENLARGED 
FIRST   THOUSAND 


NEW    YORK 

JOHN    WILEY   &    SONS 

LONDON:   CHAPMAN   &   HALL,   LIMITED 

1912 


.1 


COPYRIGHT,  1894,  1902,  1908,  1912, 

BY 
FREDERICK   P.  SPALDING 


SCIENTIFIC    PRESS 
RT    DRUMMOND   AND    COMPANY 
BROOKLYN,    N.  Y. 


PREFACE. 


SUCCESSFUL  practice  in  the  construction  of  highways 
must  depend  upon  correct  reasoning  from  elementary 
principles  in  each  instance  rather  than  upon  following 
definite  rules  or  methods  of  construction. 

The  aim  of  this  book  is  to  give  a  brief  discussion, 
from  an  engineering  standpoint,  of  the  principles 
involved  in  highway  work,  and  to  outline  the  more 
important  systems  of  construction,  with  a  view  to 
forming  a  text  which  may  serve  as  a  basis  for  a  syste- 
matic study  of  the  subject. 

Details  and  statistics  of  particular  examples  have 
for  the  most  part  been  excluded  as  undesirable  in  a 
book  of  this  character.  Such  information  is  available 
in  many  forms  for  those  having  the  necessary  elemen- 
tary training  and  experience  to  enable  them  to  properly 
use  it. 

Considerable  space  has  been  given  to  the  location 
and  construction  of  country  roads,  as  seemed  proper 
in  view  of  the  present  general  public  interest  in  the 
matter,  and  the  probable  development  of  this  new  field 
of  activity  in  engineering  work.  The  improvement  of 
our  common  roads  must  come  through  transferring 
such  work  to  the  charge  of  those  who  make  it  a  profes- 
sion, and  not  through  teaching  the  public  how  roads 
should  be  constructed. 

F.  P.  S. 
ITHACA,  N.Y.,  July,  1894. 

263616 

iii 


NOTE   TO    FOURTH   EDITION. 


DURING  the  past  few  years,  advances  in  the  methods 
employed  in  the  construction  and  maintenance  of  high- 
ways have  been  very  rapid.  Changes  in  the  character 
of  traffic,  due  to  the  introduction  of  automobiles,  have 
presented  new  problems,  while  modifications  in  the 
standards  of  life,  both  in  city  and  country,  render  the 
old  methods  no  longer  satisfactory  to  the  public.  These 
changed  conditions  have  caused  more  careful  and  scientific 
study  of  materials,  and  resulted  in  the  use  of  more  efficient 
methods,  and  in  the  development  of  new  types  of  con- 
struction. 

In  preparing  the  third  edition,  in  1908,  it  was  found 
necessary  to  practically  rewrite  the  entire  book.  In  the 
present  edition,  new  chapters  are  added  on  Bituminous 
Macadam  and  Concrete  Pavements,  while  the  chapters 
on  Brick,  Asphalt  and  Wood  Pavements  have  been 
considerably  modified.  The  size  of  the  book  has  neces- 
sarily expanded  considerably  beyond  its  former  limits. 

F.  P.  S. 
COLUMBIA,  Mo,,  June,  1912. 


CONTENTS. 


CHAPTER   I, 
ROAD  ECONOMICS  AND  MANAGEMENT. 

PAGB 

Art.     i.    Object  of  Roads i 

2.  Resistance  to  Traction ,  3 

3.  Tractive  Power  of  Horses 8 

4.  Benefits  Derived  from  Good  Roads 10 

5.  Cost  of  Wagon  Transportation • 12 

6.  Economic  Value  of  Road  Improvement 16 

7.  Sources  of  Revenue  for  Road  Improvement 19 

8-    Systems  of  Road  Management 22 

CHAPTER   II. 
DRAINAGE  OF  STREETS  AND  ROADS. 

Art.     9.    Necessity  for  Drainage 26 

10.  Surface  Drainage 28 

11.  Sub-drainage 29 

12     Tile  Drains 33 

13.  Stone  Drains 36 

14.  Culverts 38 

15-    Concrete  Culverts 45 

CHAPTER    III. 
LOCATION  OF  COUNTRY  ROADS, 

Art    1 6    Considerations  Governing  Location . 49 

1 7.  Length  of  Road 52 

18.  Rise  and  Fall 55 

19.  Rate  of  Grade 57 

20.  Examination  of  Country 59 

21.  Placing  the  Line.  ...    r...  63 

22.  Comparison  of  Routes  65 

23.  Changing  Existing  Locations 69 

vii 


Vlii  CONTENTS. 

CHAPTER   IV. 
IMPROVEMENT  AND  MAINTENANCE  OF  COUNTRY  ROADS. 

PAGE 

Art.    24.    Nature  of  Improvements. , . 72 

25.  Grade  and  Cross-section , 73 

26.  Earthwork 9 ................  79 

27.  Earth  Roads 85 

28.  Gravel  Roads .......... 93 

29.  Oiled  Roads .  96 

30.  Sand-clay  Roads 107 

31.  Miscellaneous  Roads .  in 

32.  Width  of  Tires 115 

CHAPTER   V. 
BROKEN-STONE  ROADS. 

Art.  33.  Definition 117 

34.  Macadam  Roads 118 

35.  Telford  Foundations 120 

36.  Rocks  for  Road  Building 124 

37.  Methods  of  Testing  Stone 137 

38.  Road  Metal 146 

39.  Compacting  the  Road 151 

40.  Thickness  of  Road  Covering 153 

41.  Maintenance  of  Broken-Stone  Roads 156 

42.  Dust  Preventives 159 


CHAPTER  VI. 
BITUMINOUS  MACADAM  ROADS. 

Art.  43.  Types  of  Bituminous  Macadam 168 

44.  Bituminous  Materials 171 

45.  Petroleums 174 

46.  Solid  Native  Bitumens 177 

47.  Tar  Products 1 79 

48.  Tests  for  Bituminous  Materials 182 

49.  Surface  Treatment 192 


CONTENTS.  IX 

PAGE 

ART.  50.  Penetration  Method 197 

51 .  Mixing  Method 202 

52.  Selection  of  Bituminous  Materials 204 


CHAPTER  VII. 

FOUNDATIONS  FOR  PAVEMENTS. 

Art.  53.  Preparation  of  Road-bed 209 

54.  Trenches  in  Streets 210 

55.  Purpose  of  Foundation 212 

56.  Bases  of  Gravel  and  Broken  Stone 214 

57.  Concrete  Bases 214 

58.  Bituminous  Foundations 217 

59.  Miscellaneous  Foundations 218 

60.  Choice  of  Foundations 220 

CHAPTER  VIII. 

BRICK  PAVEMENTS. 

Art.  61.  Paving  Brick 221 

62.  Tests  for  Paving  Brick 229 

63.  Construction  of  Brick  Pavements 249 

64.  Filling  of  Joints 255 

65.  Maintenance  of  Brick  Pavements 261 


CHAPTER  IX. 

ASPHALT  PAVEMENTS. 

Art.  66.  Asphalt 263 

67.  Asphaltic  Cement 271 

68.  Test  for  Asphaltic  Cement 273 

69.  Surface  Mixtures 284 

70.  Construction  of  Sheet  Pavement 291 

7 1 .  Asphalt  Blocks 296 

72.  Maintenance  of  Asphalt  Pavements 298 

73.  Bitulithic  Pavements 301 


X  CONTENTS. 

CHAPTER  X. 
WOOD-BLOCK  PAVEMENTS. 

PAGE 

Art.  74.  Types  of  Wood-block  Pavement 307 

75.  Wood  Blocks 311 

76.  Treatment  of  Wood  Blocks 315 

77.  Tests  for  Wood  Blocks 321 

78.  Construction  of  Wood-block  Pavements 322 

79.  Maintenance  of  Wood-block  Pavements 326 

CHAPTER  XI. 

STONE-BLOCK  PAVEMENTS. 

Art.  80.  Stone  for  Pavements 328 

81.  Cobblestone  Pavements 330 

82.  Belgian  Blocks 331 

83.  Granite  and  Sandstone  Blocks 332 

84.  Construction 334 

85.  Stone  Trackways 337 

CHAPTER  XII. 
CONCRETE  PAVEMENTS. 

Art.  86.  Concrete  as  Surface  Material 339 

87.  Portland  Cement 340 

88.  Portland  Cement  Mortar 343 

89.  Portland  Cement  Concrete 345 

90.  Mortar  Surfaced  Pavements 348 

91.  Monolithic  Concrete  Pavements 354 

92.  Grouted  Concrete  Pavements 356 

CHAPTER  XIII. 
CITY  STREETS. 

Art.  93.  Arrangement  of  City  Streets 357 

94.  Width  and  Cross-section 362 

95.  Street  Grades 367 

96.  Street  Intersections 370 


CONTENTS.  XI 


Art.  97.  Footways 372 

98.  Curbs  and  Gutters 378 

99.  Crossings 384 

100.  Street-railway  Track 385 

101.  Trees  for  Streets 398 

102.  Selection  of  Pavements 399 

103.  Sources  of  Revenue  for  Street  Improvement 406 


ROADS   AND   PAVEMENTS. 

CHAPTER  I. 

ROAD  ECONOMICS  AND  MANAGEMENT. 
ART.  i.     OBJECT  OF  ROADS. 

THE  primary  object  of  a  road  or  street  is  to  provide 
a  way  for  travel,  and  for  the  transportation  of  goods 
from  one  place  to  another.  The  facility  with  which 
traffic  may  be  conducted  over  any  given  road  depends 
upon  the  resistance  offered  to  the  passing  of  vehicles 
by  the  surface  or  the  grades  of  the  road,  as  well  as 
upon  the  freedom  of  movement  allowed  by  the  width 
and  form  of  the  roadway.  In  order  that  a  road  may 
offer  the  least  resistance  to  traffic,  it  should  have  as 
hard  and  smooth  a  surface  as  possible,  while  affording 
a  good  foothold  to  horses,  and  should  be  so  located  as 
to  give  the  most  direct  route  with  the  least  gradients. 

The  expediency  of  any  proposed  road  construction 
or  improvement  depends  upon  its  desirability  as  affect- 
ing the  comfort,  convenience,  and  health  of  residents 
of  the  locality,  and  also  upon  its  economic  value,  which 
is  largely  determined  by  its  cost  and  durability,  as  well 
as  upon  the  facility  it  gives  for  the  conduct  of  traffic. 

The  desirability  of  a  road  surface  for  any  particular 
use  depends  both  upon  its  fitness  for  the  service 
required  of  it  and  upon  its  durability  in  use. 


$'t     :  A;TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Upon  a  country  road,  the  problem  of  improvement 
ordinarily  consists  simply  in  providing  the  hardest  and 
most  durable  surface  consistent  with  an  economical 
expenditure  of  available  funds,  the  object  being  to 
lighten  the  cost  of  transportation  by  reducing  the 
resistance  to  traction,  and  to  render  travel  easy  and 
comfortable. 

Upon  city  streets,  however,  several  other  factors 
may  be  of  importance  in  the  design  of  highway 
improvements. 

The  comfort  both  of  those  using  the  street  and  of 
the  occupants  of  adjoining  property  will  be  largely 
affected  by  the  freedom  of  the  surface  from  noise  and 
dust. 

The  safety  of  the  pavement  in  use,  its  effect  upon 
the  health  of  residents  of  the  locality,  and  its  economic 
value  must  in  each  case  be  considered. 

To  adjust  to  the  best  advantage  these  various  ele- 
ments, frequently  quite  discordant  with  each  other,  is 
a  matter  which  can  only  be  accomplished  by  the  exer- 
cise of  good  judgment.  Local  conditions  and  necessi- 
ties must  always  be  considered  —  such  as  the  difficulties 
of  drainage,  the  availability  of  various  materials,  the 
nature  of  the  traffic  to  be  carried,  and  the  needs  of  the 
business  or  property  interests  of  the  neighborhood. 
Thus,  for  heavy  hauling  of  a  large  city,  the  durability 
and  resistance  to  wear  of  the  pavement  may  be  the 
paramount  consideration;  for  an  office  district,  quiet 
may  be  very  important;  for  the  lighter  driving  of  a 
residence  street,  the  elements  of  comfort  and  health- 
fulness  may  properly  be  considered  as  of  greater  force 
than  the  purely  economic  ones;  while  in  all  of  the 
cases  the  necessary  limitation  of  first  cost  will  largely 
determine  what  may  or  may  not  be  done. 


ROAD   ECONOMICS  AND   MANAGEMENT.  3 

The  problem  of  the  highway  engineer,  in  designing 
works  of  this  character,  involves  the  consideration  of 
these  various  elements  and  their  proper  adjustment  to 
give  the  best  results. 

The  kinds  of  road  surface  most  commonly  employed 
are  as  follows:  For  the  streets  of  cities  and  towns, 
pavements  of  stone  blocks,  brick,  asphalt,  and  wood; 
for  suburban  streets  and  important  country  roads, 
macadam  and  gravel  surfaces;  for  ordinary  country 
roads  in  general,  surfaces  of  earth  or  gravel. 

ART.  2.     RESISTANCE  TO  TRACTION. 

The  resistance  to  traction  of  a  vehicle  on  a  road 
surface  may  be  divided  into  three  parts:  axle  friction, 
rolling  resistance,  and  grade  resistance. 

Axle  friction  varies  with  the  nature  of  the  bearing 
surfaces,  and  for  vehicles  of  similar  construction  is 
directly  proportional  to  the  load.  It  is  entirely  inde- 
pendent of  the  nature  of  the  road  surface. 

Rolling  resistance  is  of  two  kinds:  that  due  to  irregu- 
larities in  the  surface  of  the  road,  and  that  of  a  wheel 
to  rolling  upon  a  smooth  surface,  sometimes  called 
rolling  friction. 

The  resistance  due  to  an  inequality  in  the  road  sur- 
face is  the  horizontal  force  necessary,  at  the  axle,  to 
raise  the  weight  upon  the  wheel  to  the  height  of  the 
obstacle  to  be  passed.  Thus  (Fig.  i),  by  the  principle 

of  the  lever,  P  =  W-- 
a 

For  small  inequalities,  this  resistance  will  be  approxi- 
mately inversely  as  the  diameter  of  the  wheel.  The 
effect  of  small  irregularities  in  the  surface,  however, 
is  due  more  to  the  shocks  and  concussions  produced  by 


4  A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

them  than  to  the  direct  lifting  action  of  the  obstacle, 
and  the  resistance  due  to  uneven  surface  is  greater  at 
high  than  at  low  velocities. 

Rolling  friction  is  probably  due 
for  the  most  part  to  the  compres- 
sibility of  the  surface  of  the  road, 
which  permits  the  wheel  to  indent 
it  to  some  extent.  The  wheel  is 
thus  always  forcing  a  wave  of  the 
surface  before  it,  or  climbing  an 
inclination  caused  by  its  weight 
upon  the  road  surface. 

Size  of  Wheels.  —  The  resistance  to  traction  varies 
for  wheels  of  differing  diameters,  being  less  for  large 
than  for  small  wheels.  The  experiments  of  M.  Morin, 
in  France,  seemed  to  indicate  that  the  resistance  varies 
inversely  as  the  diameter.  Other  experiments  have 
indicated  a  less  variation,  approximately  as  the 
square  root  of  the  diameter,  while  Mr.  D.  K.  Clark 
(Roads  and  Streets,  by  Law  and  Clark;  London, 
1890)  concludes,  from  a  mathematical  discussion  based 
upon  the  assumption  that  the  material  of  the  surface 
is  homogeneous  and  the  pressure  proportional  to  the 
depth  of  penetration,  that  the  resistance  to  traction 
is  inversely  as  the  cube  root  of  the  diameter  of  the 
wheel.  The  experiments  of  Mr.  Mairs  (Bulletin,  Uni- 
versity of  Missouri  Agricultural  Experiment  Station, 
1902)  indicate  that  tractive  resistance  is  somewhat 
less  with  large  than  with  small  wheels,  being  nearly 
inversely  as  the  square  root  of  the  diameter,  but  as 
might  be  expected,  differing  somewhat  for  different 
road  surfaces. 

For  practical  purposes  it  may  be  considered  that,  for 
wheels  of  ordinary  sizes  used  on  road  vehicles,  the 


ROAD   ECONOMICS   AND   MANAGEMENT.  5 

rolling  resistances  are  equal  to  the  load  multiplied 
by  a  coefficient  which  depends  upon  the  nature 
and  condition  of  the  road  surface,  although  these 
coefficients  are  somewhat  affected  by  the  sizes  of  the 
wheels. 

Width  of  Tire.  —  The  effect  upon  tractive  resistance 
of  the  width  of  tire  upon  the  wagon  wheels  depends 
upon  the  character  of  the  surface  upon  which  the  wheel 
is  rolling.  In  a  series  of  experiments  at  the  University 
of  Missouri  Agricultural  Experiment  Station  in  1897, 
it  was  found  that  wide  tires  considerably  diminished 
tractive  resistance  upon  broken  stone  and  gravel  roads, 
and  upon  earth  roads  in  good  condition,  but  upon 
muddy  roads  or  when  a  hard  road  is  covered  with  deep 
dust  the  resistance  is  greater  for  wride  tires.  The  wide 
tire  also  has  considerable  advantage  upon  plowed 
land  or  sod,  not  cutting  in  so  deeply. 

Speed  of  Travel.  —  Tractive  resistances  are  somewhat 
greater  at  high  than  at  low  velocities.  This  difference 
is  very  slight  on  earth  roads  in  good  condition  or  on 
smooth  pavements;  but  on  rough  pavements  where  con- 
cussions take  place  the  resistance  increases  rapidly  as 
the  speed  becomes  greater. 

Road  Surface.  —  Many  experiments  have  been  made 
for  the  purpose  of  determining  the  tractive  force 
required  for  a  given  load  upon  various  road  surfaces. 
The  results  show  somewhat  wide  variations,  as  would 
be  expected  when  the  many  elements  that  may  affect 
them  are  considered.  The  following  table  shows  a 
few  average  results,  which  will  give  some  idea  of  the 
relative  resistances  of  various  surfaces  and  of  the 
advantage  to  be  derived  from  a  smooth  and  well-kept 
road  surface: 


A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 
TRACTIVE    RESISTANCES    ON    VARIOUS    SURFACES. 


Character  of  Road. 


Resistance  per 

Ton, 
Pounds. 


Earth  Roads  —  in  fair  condition  .  .  . 

dry  and  hard 

Macadam  —  very  good 

ordinary 

poor 

Granite  block  pavement  —  good .... 
ordinary . 

Brick  pavement 

Wood  block  pavement 

Asphalt  pavement 


100  to  175 
60  to  125 
25  to  50 
40  to  ioo 

75  to  J5° 
25  to  50 
40  to 

20  tO 

25  to 

20  tO 


80 


70 


The  resistance  upon  asphalt  is  greater  at  high  than 

at  low  temperatures. 

Grade  Resistances.  —  Tractive  resistance  due  to  grade 

is  independent  of  the  nature  of  the  road  surface  or  of 

the  size  of  the  wheels. 
It  is  equal  to  the  load 
multiplied  by  the  sine 
of  the  angle  made  by 
the  grade  with  the  hor- 
izontal. Thus  in  Fig.  2 
the  tractive  force  P, 
FlG-  2-  due  to  the  grade,  is  the 

force  necessary  to  pre- 

vent the  wheel  from  rolling  down  the  slope  under  the 

action  of  the  weight  W,  or  it  is  the  component   of  W 

parallel  to  the  slope  ac. 


ac 


Grades  are  ordinarily  expressed  in  terms  of  rise  or 
fall  in  feet  per  hundred,  or  as  percentage  of  horizontal 
distance. 


.      ROAD    ECONOMICS  AND   MANAGEMENT.  7 

For  all   ordinary  cases  of  small  inclinations  ab  is 
approximately  equal  to  ac,  and  we  may  take 


- 

ab 

or  the  tractive  force  necessary  to  overcome  any  grade 
equals  the  load  multiplied  by  the  percentage  of  grade. 
The  total  tractive  force  necessary  to  haul  a  load  up 
an  inclined  road  equals  the  sum  of  the  force  necessary 
to  haul  the  load  upon  the  same  surface  when  level  and 
the  force  necessary  to  overcome  the  grade  resistance. 
Thus,  if  we  wish  to  find  the  tractive  effort  necessary  to 
haul  a  load  of  2  tons  up  a  grade  of  3  ft.  in  100  over  a 
good  macadam  road.  Taking  the  resistance  of  the 
road  surface  when  level  at  60  Ibs.  per  ton,  we  have  for 
the  total  resistance 

R  =  2  X  60  +  4000  X  Y§-0-  =  240  Ibs. 

In  going  down  the  grade,  the  force  due  to  grade 
becomes  a  propelling  force,  and  the  tractive  effort 
required  is  the  difference  between  the  surface  resist- 
ance and  grade  force.  In  case  the  grade  force  be  the 
greater,  the  resulting  tractive  force  becomes  negative, 
or  it  will  be  necessary  to  apply  the  force  as  a  resistance 
to  prevent  acceleration  of  the  velocity  in  the  descent. 

The  angle  for  which  the  tractive  force  required  for  a 
given  surface  equals  the  grade  resistance  is  called  the 
Angle  of  Repose  for  that  surface.  In  the  case  given 
above,  2  X  60  —  4000  X  -j-f-Q-  =  o,  or  the  angle  of  repose 
for  a  surface  whose  level  resistance  is  60  Ibs.  per  ton  is 
a  3  per  cent  grade.  If  a  vehicle  were  left  standing  upon 
that  inclination,  it  should  remain  standing  with  the 
forces  just  balanced.  If  it  were  started  down  the 
grade,  it  should  continue  to  move  at  a  uniform  rate, 
without  the  application  of  any  other  force. 


8  A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  3.     TRACTIVE  POWER  OF  HORSES. 

The  loads  that  a  horse  can  pull  upon  various  road 
surfaces  will  not  necessarily  be  proportional  to  the 
resistance  offered  by  the  surface  to  traction,  as  the 
tractive  force  that  the  horse  can  exert  depends  upon 
the  foothold  afforded  by  the  surface.  The  ability  of  a 
horse  to  exert  a  tractive  force  depends  upon  the 
strength  of  the  animal,  upon  his  training  for  the  par- 
ticular work,  and  whether  he  be  accustomed  to  the 
surface  upon  which  he  is  travelling.  The  work  of  dif- 
ferent animals  is  therefore  subject  to  considerable  varia- 
tions, and  only  very  rough  approximations  are  possible 
in  giving  average  values  of  the  work  a  horse  may  do 
under  differing  conditions. 

The  tractive  force  that  may  be  exerted  by  a  horse, 
at  moderate  speeds,  varies  approximately  inversely  as 
the  rate  of  speed;  or,  in  other  words,  the  power  that 
a  horse  can  exert  through  any  considerable  time  is 
nearly  constant  for  varying  velocities.  Thus  it  may 
be  assumed,  as  an  average  value,  that  a  horse  working 
regtdarly  ten  hours  per  day  can  put  forth  a  tractive 
effort  of  80  pounds  at  a  speed  of  250  feet  per  minute 
on  an  ordinary  level  road  surface. 

For  the  power  of  the  horse  we  then  have 

Power  =  force  X  velocity  =  80  X  250  =  20000     foot- 
Ibs.  per  minute. 

For  any  other  rate  of  speed,  as  200  feet  per  minute, 
we  would  have  20000  -r-  200  =  100  pounds  as  the 
tractive  force  exerted  by  the  horse. 

If  the  period  of  daily  work  be  lessened,  the  power 
that  may  be  developed  will  be  increased,  either  by 
increasing  the  load  or  the  velocity. 


ROAD  ECONOMICS  AND  MANAGEMENT.      9 

The  tractive  force  that  a  horse  is  able  to  exert 
decreases  very  rapidly  as  the  rate  of  inclination 
increases.  This  is  due  both  to  the  expenditure  of 
power  by  the  horse  in  lifting  his  own  weight  up  the 
grade,  and  to  the  less  firm  footing  on  the  inclination. 
The  effect  of  differences  in  the  foothold  afforded  by 
various  pavements  is  very  marked  in  the  loss  of  tractive 
power  upon  grades. 

In  the  table  below  are  given  the  loads  that  an 
average  horse  may  be  expected  to  continuously  haul 
up  different  inclinations,  on  various  road  surfaces,  at 
slow  speed.  These  figures,  while  of  little  value  as  an 
absolute  measure  of  what  may  be  done  in  any  par- 
ticular case,  are  of  use  as  a  rough  comparison  of  the 
relative  tractive  properties  of  different  surfaces  and 
grades.  The  effect  of  grades  upon  tractive  effort  will 
also  depend  upon  the  condition  in  which  the  surface  is 
maintained,  and  upon  the  weather.  Snow  and  ice  in 
winter,  or  the  damp  and  muddy  condition  of  some 
pavements  in  wet  weather,  have  a  very  considerable 
effect  to  diminish  tractive  power. 


LOADS  IN   POUNDS  THAT  A  HORSE  CAN  DRAW  UPON  VARIOUS 
SURFACES    AND    GRADES. 


Rate  of  Grade. 

Kinds  of  Surface. 

Level. 

i  in 

2  in 

3  in 

4  in 

Sin 

10  in 

15  in 

IOO. 

IOO. 

IOO. 

IOO. 

IOO. 

IOO. 

IOO. 

Earth  road  —  good  

3000 

2400 

2000 

1000 

1400 

I2OO 

800 

300 

poor  
Broken-stone  —  good  

1300 
4000 

IIOO 

2700 

900 

2OOO 

700 
1000 

000 

1400 

500 

1200 

400 
700 

150 
200 

poor  

1600 

IIOO 

800 

600 

500 

45° 

250 

IOO 

Stone  Blocks  —  good  

6000 

4500 

3300 

2700 

2200 

1700 

QOO 

400 

poor 

2QOO 

•?  2OO 

I  7OO 

I4.OO 

IIOO 

A?Q 

Asphalt  —  clean  and  dry  .  .  . 

8000 

4000 

2500 

1800 

1300 

1000 

<K>IJ 

4OO 

IO         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

In  general,  the  tractive  effort  that  a  horse  may  exert 
is  approximately  proportional  to  the  weight  of  the 
horse  and  the  averages  above  given  correspond  to  light 
animals;  many  horses  are  capable  of  exerting  double 
the  pull  mentioned. 

A  horse  may  frequently  exert  for  a  short  time  a 
tractive  force  about  double  that  which  he  can  exert 
continuously;  hence,  when  short  grades  occur  steeper 
than  the  general  grades  of  the  road,  loads  may  often 
be  taken  over  them  much  heavier  than  could  be  carried 
if  the  steeper  grade  prevailed  upon  the  road. 

On  ordinary  country  roads  in  dry  weather  the 
amount  of  load  that  can  be  hauled  is  usually  deter- 
mined rather  by  the  grades  than  by  the  nature  of  the 
surface.  Unless  the  gradients  are  very  light  the 
amount  of  load  that  can  be  carried  on  a  broken-stone 
surface  does  not  differ  greatly  from  what  may  be  taken 
on  a  dry  and  hard  earth  road.  In  improving  a  road 
by  substituting  a  hard  surface  for  a  surface  of  earth 
the  gradients  and  location  should  therefore  always  be 
carefully  studied,  with  a  view  to  deriving  the  full 
practical  benefit  from  the  hard  surface  in  the  light 
traction  that  it  may  require  with  easy  ruling  gradients. 

ART.  4.     BENEFITS  DERIVED  FROM  GOOD  ROADS. 

The  condition  of  the  public  highways  is  a  matter  of 
the  most  vital  interest  to  any  rural  community.  Upon 
it  depends  largely  the  social  life  and  enjoyment  of  the 
people  living  in  the  country  as  well  as  the  ability  to 
market  the  products  of  the  farm  to  the  best  advantage. 

In  nearly  all  parts  of  the  country  the  roads  are  fairly 
good  during  a  portion  of  the  year;  but  there  is  also 
usually  a  period  when  they  are  very  bad,  in  very 


ROAD  ECONOMICS  AND  MANAGEMENT.      II 

many  localities  becoming  practically  impassable.  The 
improvement  of  the  road  surfaces  and  the  use  of 
systematic  maintenance  would  make  the  roads  better 
at  all  times,  making  it  possible  to  haul  larger  loads 
over  them  and  rendering  them  more  pleasant  to  travel; 
but  the  most  important  object  of  road  improvement  is 
to  eliminate  the  period  when  roads  are  not  in  condition 
to  use  and  make  it  possible  to  drive  upon  them  and 
haul  loads  over  them  at  all  times. 

The  benefits  of  good  roads  may  be  classified  as  social, 
educational,  and  financial.  They  promote  social  inter- 
course among  the  residents  of  a  country  district  by 
making  travel  easy  and  pleasant.  Where  the  roads 
become  impassable  during  a  portion  of  the  year,  the 
residents  are  practically  isolated  at  the  period  of 
greatest  leisure  and  lose  that  intercourse  with  their 
neighbors  which  is  a  most  important  means  of  enjoy- 
ment  and  development.  Attendance  at  church  and 
public  meetings  is  facilitated  by  good  roads.  There 
are  many  localities  where  the  condition  of  the  roads 
practically  closes  the  churches  during  a  considerable 
portion  of  each  year,  and  in  some  instances  they  have 
been  so  deserted  on  this  account  as  to  be  abandoned. 
The  rural  mail  delivery  also  depends  for  its  efficiency 
upon  the  good  condition  of  the  roads. 

The  consolidation  of  rural  schools  and  establish- 
ment of  rural  high  schools,  made  possible  by  good  roads, 
is  an  important  advance  in  educational  methods,  and 
places  rural  communities  more  nearly  on  an  equality 
with  the  cities  in  educational  advantages  offered  to 
children. 

Roads  which  can  be  traveled  all  the  year  admit  of 
marketing  the  products  of  the  farm  at  any  time  which 
may  be  most  advantageous,  enabling  the  farmer  to  take 


12          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

advantage  of  favorable  market  conditions  and  prices, 
or  to  transport  his  products  at  a  convenient  season, 
when  he  can  do  the  work  without  interference  with 
other  duties  of  men  and  teams. 

The  condition  of  the  highways  has  also  considerable 
effect  upon  the  business  of  the  towns  into  which  they 
may  lead;  where  they  are  uniformly  good  throughout 
the  year  mercantile  business  will  be  better  distributed 
between  different  seasons,  and  a  larger  volume  of 
business  will  be  transacted.  The  same  effect  is  pro- 
duced upon  railway  transportation.  Congestion  in  rail- 
wray  business  and  scarcity  of  cars  is  frequently  the 
result  of  the  hurried  marketing  of  crops  to  take  advan- 
tage of  a  good  condition  of  the  wagon  roads,  and  a 
much  better  distribution  of  business  might  be  obtained 
through  an  improved  condition  of  the  highways. 
The  area  tributary  to  a  town  or  a  railway  may  also 
frequently  be  considerably  extended  by  road  improve- 
ment. 

The  greatest  benefits  derived  from  good  roads  are  in 
the  increased  comfort,  convenience,  and  pleasure  of  the 
people  living  near  them,  and  in  the  social  and  edu- 
cational advantages  which  they  make  possible  and 
which  add  greatly  to  the  attractiveness  and  happiness 
of  rural  life. 

ART.  5.     COST  OF  WAGON  TRANSPORTATION. 

The  effect  of  bad  roads  upon  the  cost  of  wagon 
transportation  has  been  the  subject  of  much  discussion 
and  many  estimates  have  been  made  which  have 
arrived  at  widely  different  conclusions.  Many  of 
these  discussions  have  failed  to  take  account  of  all  the 
factors  entering  into  the  problem  and  have  arrived  at 


ROAD  ECONOMICS  AND  MANAGEMENT.      1 3 

wildly  extravagant  results.  Efforts  have  been  made 
by  the  Road  Inquiry  Office  of  the  United  States 
Department  of  Agriculture  to  collect  statistics  con- 
cerning the  volume  and  cost  of  hauling  farm  produce  to 
market  for  the  whole  United  States.  These  statistics 
include  estimates  of  the  average  length  of  haul  and  the 
cost  per  ton-mile,  with  a  view  to  basing  upon  them  some 
conclusion  as  to  the  saving  to  the  country  in  general 
which  would  result  from  the  improvement  of  the  roads 
so  that  larger  loads  may  be  carried,  and  less  labor  be 
required  for  moving  the  crops.  Statistics  of  this  kind 
are  very  difficult  to  gather,  being  altogether  dependent 
upon  the  judgment  of  the  man  who  collects  them  in 
each  locality,  and  representing  only  very  approximately 
the  average  conditions.  They  are  of  value  as  giving 
information  concerning  the  traffic  to  which  the  roads 
are  subject  in  various  localities  and  the  need  for  road 
improvement,  but  they  do  not  contain  data  upon  which 
any  reasonable  estimate  can  be  based  of  the  actual 
saving  which  might  be  effected  by  road  improvement. 
Such  general  estimates  are  not  of  any  particular  value 
other  than  that  of  showing  something  of  the  size  of  the 
problem  when  applied  to  the  wrhole  country. 

Many  palpably  erroneous  and  exaggerated  estimates 
of  the  saving  in  cost  of  transportation  by  road  improve- 
ment have  been  published  and  have  often  seriously 
injured  the  cause  of  good  roads.  They  aim  to  show 
the  large  saving  wrhich  may  be  effected  by  the  farmer 
through  reducing  the  cost  of  moving  his  crops  to 
market,  but  their  fallacies  are  evident  to  the  farmer 
who  reads  them  and  applies  them  to  his  own  conditions, 
and  in  many  instances  lead  him  to  doubt  the  good 
faith  of  the  whole  movement  for  good  roads.  These 
estimates  commonly  treat  the  subject  as  though  the 


14          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

whole  of  the  crops  were  hauled  to  market  in  full  loads 
by  teams  kept  by  the  farmers  for  that  purpose  alone, 
and  which  could  be  dispensed  with  if  the  roads  were  so 
improved  as  to  require  a  less  number  of  loads,  and  con- 
sequently less  teams  to  transport  the  crops,  which  is 
clearly  not  the  case. 

The  effect  of  the  condition  of  the  highways  upon 
the  cost  of  wagon  transportation  depends  upon  the 
character  of  the  traffic.  Where  this  consists  of  the 
transportation  of  some  product  which  is  hauled  in  full 
loads,  with  teams  which  are  employed  for  this  purpose 
only,  the  cost  of  transportation  is  readily  ascertained, 
and  the  saving  due  to  any  improvement  which  increases 
the  load  carried  by  each  team  may  be  found  by  esti- 
mating the  cost  of  the  decreased  number  of  teams 
required.  If  an  earth  road  in  poor  condition  be  re- 
placed by  a  good  macadam  surface,  the  load  which 
can  be  taken  over  the  road  may  easily  be  doubled  if 
upon  light  gradients,  while  where  a  traffic  of  this 
character  must  be  taken  over  an  earth  road  in  bad  or 
muddy  condition,  the  construction  of  an  improved 
road  surface  may  result  in  loads  four  or  five  times  as 
heavy  as  before.  In  this  case  the  number  of  teams  is 
inversely  proportional  to  the  maximum  load  which  may 
be  hauled  and  the  cost  is  proportional  to  the  number 
of  teams.  This  condition  soemtimes,  though  rarely, 
occurs  upon  wagon  roads,  the  traffic  usually  being  of  a 
mixed  character,  with  varying  percentages  carried  in 
full  loads,  and  with  teams  kept  for  other  purposes  and 
only  incidentally  used  for  transportation  upon  the 
roads. 

For  the  purpose  of  estimating  the  cost  of  trans- 
portation upon  ordinary  country  roads  it  is  necessary 
to  separate  the  traffic  into  classes  and  determine  what 


ROAD  ECONOMICS  AND  MANAGEMENT.  1$ 

portion  of  it  is  carried  in  full  loads.  This  is  always  a 
matter  of  difficulty  where  the  traffic  is  varied  and 
can  only  be  done  in  a  very  roughly  approximate 
manner.  The  light  portion  of  the  traffic  will,  of  course, 
be  benefited  by  improved  roads,  but  the  saving  in 
cost  of  conducting  the  traffic,  while  existent,  is 
usually  comparatively  small  and  practically  indeter- 
minate. It  consists  in  saving  time  of  men  and  teams 
through  greater  speed  of  travel,  and  in  less  wear 
upon  teams  and  vehicles.  When  such  traffic  must  be 
conducted  over  muddy  and  bad  roads  these  items 
may  be  of  considerable  importance,  although  they  can 
not  be  evaluated,  but  commonly  they  are  of  slight 
importance. 

The  heavy  portion  of  the  traffic  is  more  directly 
affected  by  the  character  of  the  roads  over  which  it 
passes.  This  traffic  is  carried  in  full  loads,  which  are 
limited  in  amount  by  the  condition  and  gradients  of  the 
roads.  In  some  localities  this  constitutes  the  main 
portion  of  the  traffic;  in  others  it  is  a  comparatively 
small  part  of  the  whole.  No  generalization  concern- 
ing the  value  of  road  improvement  as  reducing  the  cost 
of  transportation  can  therefore  be  made  with  any 
approach  to  accuracy;  but  in  a  particular  instance  where 
data  is  obtainable  concerning  the  heavy  traffic,  it  is 
possible  to  roughly  estimate  the  saving  in  labor  of 
transportation  through  road  improvement.  If  we 
can  determine  the  cost  of  using  teams  for  this  purpose, 
an  approximate  estimate  may  also  be  made  of  the 
saving  in  cost  of  transportation  through  such  improve- 
ment. The  cost  of  using  teams  for  highway  trans- 
portation is  often  difficult  to  obtain  on  account  of  the 
fact  that  such  teams  are  commonly  kept  for  other 
purposes  and  only  incidentally  used  for  road  work, 


1 6          A  TEXT-BOOK  ON  ROADS  AN*D  PAVEMENTS. 

and  in  some  instances  it  is  possible  that  the  trans- 
portation is  done  when  there  is  no  other  work  which 
could  be  done  by  the  team.  In  general,  however,  it  is 
fair  to  assume  that  the  cost  of  the  work  is  proportional 
to  its  amount,  and  that  if  the  teams  were  not  employed 
in  transportation  on  the  highway,  they  would  be  other- 
wise usefully  engaged.  In  estimating  the  cost  of  work 
of  teams,  the  actual  cost  to  the  farmer  of  keeping  the 
team  should  be  used  and  not  the  rental  value  of  teams 
in  the  vicinity. 

A  careful  examination  of  the  local  conditions  sur- 
rounding the  traffic  is  essential  to  any  reasonable 
estimate  of  saving  to  be  effected  in  cost  of  transpor- 
tation upon  highways.  Such  estimates  are  not  of 
much  value  at  best  as  giving  actual  amount  of  savings, 
but  studies  of  this  kind  may  be  of  value  in  giving  a 
better  conception  of  the  economics  of  the  good  roads 
problem. 

ART.  6.       ECONOMIC  VALUE  OF  ROAD 
IMPROVEMENT. 

The  value  of  a  road  improvement  to  a  community 
and  the  amount  of  money  that  may  reasonably  and 
profitably  be  expended  in  the  construction  and  main- 
tenance of  common  roads  is  a  subject  the  discussion  of 
which  leads  different  persons  to  widely  different  con- 
clusions, depending  upon  the  point  of  view  and  the  data 
assumed.  Any  improvement,  either  in  position  or 
surface,  that  has  the  effect  of  increasing  the  loads  that 
may  be  taken  over  a  road  by  a  given  power  lessens  the 
number  of  loads  necessary  to  carry  the  traffic,  and 
effects  a  saving  in  time  and  labor  of  men  and  teams, 
which  may  reasonably  be  considered  to  have  the  same 


ROAD  ECONOMICS  AND  MANAGEMENT.      17 

money  value  as  the  time  used  in  the  work.  This  has 
been  discussed  in  Art.  5  and  is  the  most  direct  and 
obvious  financial  gain  which  may  result  from  road 
improvement. 

Saving  in  cost  of  transportation  is  not,  however, 
the  most  important  advantage  to  be  gained  by  road 
improvement,  and  if  it  were  the  only  one,  in  many 
instances,  the  expenditure  of  money  necessary  to 
secure  better  roads  could  not  be  justified  as  econom- 
ically profitable. 

It  is  in  wet  and  muddy  weather  that  improved 
surfaces  have  their  chief  advantage  over  earth  roads, 
and  the  main  object  of  introducing  hard  and  imperme- 
able surfaces  is  to  eliminate  the  period  when  ordinary 
earth  roads  are  apt  to  be  muddy  and  practically 
useless  for  the  purposes  of  transportation,  and  to 
substitute  a  road  that  may  be  used  at  any  season. 
Systematic  drainage  has  a  similar  object.  To  &  farm- 
ing community  the  economic  advantage  of  a  road 
uniformly  good  at  all  seasons  is  greater  than  might 
appear  at  first  glance.  It  may  in  many  instances 
amount  practically  to  a  saving  equal  to  nearly  the 
entire  cost  of  hauling  by  permitting  the  work  to  be 
done  at  times  when  other  work  is  impossible,  thus 
making  men  and  teams  available  for  other  duty  in 
good  weather.  The  ability  to  use  a  road  at  any 
season  is  also  of  advantage  in  the  independence  of 
weather  that  will  make  it  possible  to  take  advantage 
of  the  condition  of  the  markets  in  the  disposal  of 
produce  or  purchase  of  supplies.  These  advantages 
may  be  of  greater  or  less  importance  according  to  the 
character  of  the  traffic  carried  by  the  road.  In  general, 
while  they  are  indeterminate  and  can  not  be  expressed 
in  money  value,  they  are  evidently  of  more  economic 


18         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

importance  than  the  saving  effected  in  costs  of  trans- 
portation. 

The  nature  of  the  country  roads  affects  the  towns  to 
which  the  country  is  tributary  as  well  as  the  country 
itself.  They  directly  affect  trade  in  seasons  of  bad 
weather,  both  in  regulating  the  demand  for  supplies 
for  country  consumption  and  in  controlling  the  supply 
of  produce  which  is  available  for  market;  indirectly 
also  the  prosperity  of  a  rural  district  means  that  of  its 
trade  center.  The  improvement  of  country  roads  is, 
therefore,  of  direct  economic  value  to  towns  into  which 
they  may  lead,  but  this,  like  most  of  the  other  advan- 
tages of  good  roads,  is  dependent  upon  data  which  can 
not  be  accurately  estimated. 

All  of  these  points  must  be  considered  in  any  at- 
tempt to  arrive  at  any  proper  conception  of  the  advan- 
tages of  a  proposed  improvement.  In  any  particular 
case  the  local  interests  will  determine  the  relative 
importance  of  the  various  elements,  and  a  careful 
analysis  of  the  trade  that  does  pass  over  the  road  and 
that  would  pass  over  it  under  different  conditions  will 
enable  a  judgment  to  be  formed  as  to  the  value  of 
improvements.  The  attempt  to  base  an  estimate  of 
the  economic  value  of  a  proposed  road  improvement 
upon  the  prospect  of  direct  financial  return  is,  however, 
apt  to  be  misleading  and  to  leave  out  of  account  the 
most  important  benefits  of  such  improvements.  The 
social  and  educational  benefits  mentioned  in  Art.  4 
are  of  highest  importance  and  have  also  an  economic 
value  in  their  effect  upon  the  desirability  of  a  locality 
as  a  place  of  residence.  The  economic  importance 
of  good  roads  is  shown  in  their  effect  upon  land  values, 
which  are  largely  affected  by  them. 

The   money  spent   in   road  improvement   is  to   be 


ROAD   ECONOMICS  AND- MANAGEMENT.  1 9 

considered  as  an  investment,  which  will  return  annual 
interest  to  the  community  in  reduced  costs  of  trans- 
portation, greater  freedom  of  traffic  and  travel,  and  in 
the  increased  comfort  and  happiness  of  the  people. 

ART.  7.     SOURCES  OF  REVENUE  FOR  ROAD 
CONSTRUCTION. 

Various  methods  have  been  employed  for  securing 
the  funds  necessary  for  the  construction  and  improve- 
ment of  country  roads.  Many  of  the  earlier  roads  in 
this  country  were  toll  roads  built  by  private  capital 
and  kept  up  exclusively  by  charges  paid  by  travelers. 
Toll  roads  are  objectionable  because  the\T  impose  a 
tax  upon  all  the  traffic  of  the  road,  and  also  because 
the  cost  of  management  is  usually  large,  thus  restricting 
traffic.  They  are  conducted  for  the  purpose  of  deriving 
a  profit  from  their  operation.  They  are  gradually 
disappearing  and  should  be  dispensed  with  except 
under  exceptional  circumstances. 

District  Roads.  The  most  common  method  of  rais- 
ing funds  for  road  purposes  is  by  property  and  poll 
tax  in  small  districts.  By  this  method  a  small  poll 
tax  is  assessed  against  each  voter  of  the  district, 
paj-able  either  in  money  or  labor,  and  a  certain  property 
tax  which  is  levied  uniformly  upon  property  in  the 
district,  and  often  also  payable  in  labor  or  cash  at  the 
option  of  the  property  owner.  In  some  instances 
the  property  tax  must  be  paid  in  money,  and  in  others 
a  portion  of  it  must  be  paid  in  money.  In  some  poor 
and  sparsely  settled  localities  the  poll  tax  is  the 
principle  source  of  revenue,  and  road  work  consists 
mainly  in  working  out  the  road  tax  by  the  residents 
of  the  district. 


20         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

As  a  general  thing,  the  results  of  the  collection  of 
road  tax  in  labor  are  not  good.  Under  the  labor 
system  men  do  not  work  on  the  roads  a  sufficient 
length  of  time  to  become  expert  enough  to  do  good 
work;  many  of  them  feel  no  particular  interest  in  the 
work  and  are  only  concerned  in  getting  in  the  required 
time.  Under  this  system  it  is  not  usually  possible 
to  have  the  road  work  done  when  it  is  needed,  as  the 
convenience  of  the  laborers  must  be  considered.  In 
many  places  therefore  efforts  are  being  made  to  require 
the  payment  of  property  tax  in  money.  This  is 
highly  desirable  wherever  the  money  will  be  expended 
under  proper  management,  but  in  numerous  instances 
no  improvement  has  followed  a  change  to  the  cash 
system  because  of  a  lack  of  intelligent  control  of  the 
work. 

County  Road  Tax.  In  some  states  there  is  a  general 
property  tax  for  road  and  bridge  purposes.  This 
tax  is,  usually,  at  the  disposal  of  the  county  commis- 
sioners, or  county  court,  which  uses  it  for  the  con- 
struction and  maintenance  of  county  bridges  and  also 
for  such  road  work  as  may  be  of  special  importance  and 
of  interest  to  the  county  in  general.  Frequently 
such  funds  are  used  to  encourage  the  construction  of 
improved  roads  by  paying  a  part  of  the  cost  where 
owners  of  property  specially  benefited,  or  road  dis- 
tricts in  which  the  road  may  lie,  are  willing  to  pay 
their  share  of  the  cost.  The  maintenance  of  the 
ordinary  country  roads  usually  depends  upon  the 
district  tax,  but  a  county  road  fund  judiciously 
administered  may  do  much  to  improve  the  more  im- 
portant highways  and  thus  secure  good  roads  leading 
from  various  parts  of  the  county  to  the  market 
towns. 


ROAD   ECONOMICS   AND   MANAGEMENT.  21 

Special  Assessments.  The  laws  of  some  of  the 
states  provide  for  levying  special  assessments  against 
property  benefited  by  the  improvement  of  country 
roads.  These  laws  are  usually  permissive,  allowing 
either  the  county  court  or  majority  of  property  owners 
concerned,  by  instituting  proper  proceedings  in  court 
or  before  commissioners,  to  have  the  improvement 
made  and  assessed  upon  property  within  certain 
distances  from  the  road.  In  some  instances  the 
whole  cost  is  paid  in  this  wray;  in  others  a  part  is 
appropriated  b}^  the  county,  or  state,  or  both,  and  a 
part  is  raised  by  special  assessment. 

State  Appropriations.  Several  of  the  states  appro- 
priate state  funds,  which  may  be  used  in  assisting  in 
the  construction  of  improved  roads  throughout  the 
state.  The  amount  of  assistance  given  varies  in 
different  states;  in  New  Jersey  33  J  per  cent  of  the 
cost  is  paid  by  the  state,  56!  per  cent  by  the  county, 
and  10  per  cent  by  the  abutting  property;  in  Massachu- 
setts the  state  pays  75  per  cent,  and  the  county  25  per 
cent;  in  New  York  the  state  pays  50  per  cent,  the 
county  35  per  cent,  and  the  town  15  per  cent. 

These  various  laws  all  have  for  their  object  the 
distribution  of  cost  upon  all  interests  benefited  by 
the  work.  There  has  been  much  discussion  of  this 
subject  and  many  opinions  expressed  concerning  the 
justice  of  various  methods  of  distributing  the  cost. 
The  improvement  of  main  roads  by  local  districts 
without  outside  assistance  does  not  seem  equitable, 
and  the  use  of  general  county  tax  for  this  purpose  is 
intended  to  place  a  share  of  the  cost  upon  the  towns 
and  other  interests  not  reached  in  the  local  taxes. 
It  also  secures  a  centralized  and  usually  better  control 
of  road  work.  All  of  the  sources  of  revenue  are 


22          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

benefited  by  road  improvement  and  may  reasonably 
be  expected  to  contribute  to  its  cost,  but  the  exact 
evaluation  of  relative  benefits  is  not  possible,  and  the 
amount  each  must  pay  should  be  determined  by  con- 
sidering what  is  feasible  and  in  what  way  the  greatest 
improvement  may  be  effected. 

ART.  8.     SYSTEMS  OF  ROAD  MANAGEMENT. 

Several  different  systems  for  managing  the  work  of 
constructing  and  repairing  country  roads  have  been 
proposed  or  are  in  use  in  various  places.  These 
systems  differ  in  the  placing  of  the  control  of  the  roads 
and  in  the  methods  adopted  for  providing  funds. 

The  control  of  the  roads  under  the  various  systems 
may  be  vested  in  the  national  government,  in  the  vari- 
ous State  governments,  in  county  or  parish  organiza- 
tions or  in  townships  or  districts.  In  regard  to  the 
location  of  control  and  responsibility,  it  may  be 
remarked  that  there  are  two  points  to  be  kept  in  view. 

1st.  In  order  that  the  work  may  be  economically 
conducted,  the  section  of  country  included  under  one 
control  should  be  sufficient  to  warrant  the  permanent 
employment  of  a  man,  or  corps  of  men,  whose  business 
it  shall  be  to  continually  look  after  the  roads,  study 
their  needs,  and  systematically  conduct  their  improve- 
ment. It  should  admit  of  the  ownership  and  use  of 
labor-saving  machinery  for  the  economical  execution 
of  the  work,  but  should  not  be  large  enough  to  require 
an  elaborate  and  complicated  organization. 

2d.  The  control  of  road  work  should  be  so  arranged 
that,  as  nearly  as  possible,  all  of  the  interests  directly 
affected  by  the  condition  of  any  road  shall  have  a 
voice  in  its  management  and  contribute  to  its  support. 


ROAD    ECONOMICS   AND    MANAGEMENT.  23 

Common  roads  are  essentially  local  in  their  character 
and  are  not  usually  employed  as  lines  of  continuous 
transportation  over  any  considerable  distance.  They 
are  not,  therefore,  of  state  or  national  importance  as 
lines  of  communication,  although  as  factors  in  the 
general  welfare  of  the  people  they  must,  of  course,  like 
all  other  such  factors,  be  of  general  interest  and  concern 
to  both  state  and  nation. 

The  nation,  and  in  most  cases  in  this  country  the 
state,  is  too  large  a  unit  to  assume  direct  control  of 
road  work.  In  general,  the  interests  over  so  large  an 
area  are  so  varied,  and  the  requirements  so  different, 
as  to  prevent  a  harmonious  and  successful  organization 
of  such  work  with  a  probability  of  economical  adminis- 
tration. In  some  cases,  however,  such  control  might 
be  wise  and  proper,  and  the  recognition  of  the  impor- 
tance of  road  improvement  to  the  general  welfare  of 
the  state,  through  the  payment  by  the  state  of  a 
portion  of  the  cost  of  permanent  improvements,  has 
in  some  instances  proved  a  powerful  stimulus  to  local 
action. 

The  control  of  road  management  by  towns  and  small 
districts  is  nearly  always  inefficient  because  the  organi- 
zation is  too  small  to  support  a  proper  management 
or  provide  the  necessary  appliances  for  economic  work. 
Under  this  system  the  man  in  charge  of  the  roads  is 
usually  engaged  in  other  work;  he  is  not  a  road  engineer, 
and  can,  and  is  expected  to,  give  but  little  attention  to 
the  road  work.  This  system  of  control  is  also  usually 
unfair,  except  in  case  of  roads  intended  for  the  accom- 
modation of  the  local  district  only.  For  instance,  a 
road  passing  through  a  town  may  be  a  thoroughfare 
for  the  towns  upon  each  side.  The  principal  traffic 
may  be  this  through-trade  to  points  beyond  the  limits 


24         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

of  the  town  in  which  the  road  is  situated.  The  cost  of 
keeping  up  this  road  is  largely  due  to  outside  traffic, 
and  the  intermediate  town  should  not  be  required  to 
bear  all  the  expense  of  maintenance.  On  the  other 
hand,  the  interests  of  the  towns  whose  trade  passes 
over  the  road  are  largely  affected  by  its  nature,  and 
the  people  of  these  towns  should  be  permitted  a  voice 
in  determining  the  character  of  the  road.  Most  of  the 
more  important  roads  of  every  vicinity  pass  thus 
through  several  towns,  and  the  system  of  improvement 
by  small  districts  works  injustice  both  wa}^s  —  upon 
those  who  are  obliged  to  keep  a  road  for  the  use  of 
others  and  upon  those  who  are  obliged  to  use  a  road 
they  cannot  cause  to  be  kept  in  proper  condition. 

County  management  seems  more  successful  in  this 
country  than  any  other,  as  a  county,  or  two  counties 
combined  if  necessary,  is  usually  strong  enough  to 
secure  intelligent  management  and  homogeneous 
enough  to  have  common  interests. 

The  proper  management  of  the  common  roads  in 
any  community  requires  both  experience  and  intelli- 
gence. A  man  to  be  efficient  in  such  work  must  be 
able  to  make  or  modify  location  where  necessary, 
judge  of  the  value  of  various  materials  for  purposes  of 
construction,  determine  the  necessity  for  and  means  to 
be  adopted  for  drainage,  and  possess  the  executive 
ability  to  manage  men  and  control  scattered  work. 
The  work  in  each  locality  is  a  problem  by  itself,  to  be 
solved  by  careful  study  of  the  requirements  of  the 
community,  taking  into  account  the  local  natural 
conditions  and  available  materials  and  means. 

Several  of  the  states  have  State  Highway  Commis- 
sions, or  State  Highway  Engineers,  for  the  purpose  of 
promoting  the  improvement  of  the  country  roads. 


ROAD  ECONOMICS   AND    MANAGEMENT.  2$ 

These  commissions  are  usually  mainly  advisory  in 
character.  They  investigate  and  report  upon  the 
condition  and  needs  of  the  roads,  advise  the  local 
authorities  concerning  the  best  methods  of  construction, 
furnishing  plans  and  specifications  if  desired,  and 
control  the  expenditure  of  state  funds  applied  to  road 
purposes.  These  commissions  have  accomplished  much 
in  the  way  of  improving  existing  conditions  in  several 
states,  and  have  done  much  toward  creating  sentiment 
favorable  to  the  expenditure  of  funds  for  such  work. 
In  Missouri  the  new  road  law  provides  a  combined 
county  and  district  management.  It  creates  the  office 
of  County  Road  Engineer  to  be  appointed  by  the 
county  court.  The  County  Engineer  has  direct  charge 
of  the  expenditure  of  the  general  county  road  and  bridge 
fund,  and  all  district  road  supervisors  in  the  county 
are  required  to  report  to  him  and  to  conduct  their  work 
under  his  general  direction.  The  purpose  is  to  provide 
a  competent  central  authority  in  each  county,  without 
changing  the  existing  division  into  road  districts. 
There  is  also  a  State  Highway  Engineer  whose  duties 
are  to  advise  with  the  local  authorities,  and  to  dis- 
tribute and  control  the  expenditure  of  state  appro- 
priations in  aid  of  road  improvement. 


CHAPTER   II. 

DRAINAGE  OF  ROADS  AND  STREETS. 
ART.  9.     NECESSITY  FOR  DRAINAGE. 

THE  road-bed,  usually  formed  of  the  natural  earth 
over  which  the  road  or  pavement  is  to  be  constructed, 
must  always  carry  the  loads  which  come  upon  the 
road  surface.  Where  an  artificial  road  surface  or 
pavement  is  employed,  the  earth  road-bed  is  protected 
from  the  wrear  of  the  traffic,  and  the  wheel  loads  com- 
ing upon  the  surface  are  distributed  over  a  greater 
area  of  the  road-bed  than  if  the  loads  come  directly 
upon  the  earth  itself;  but  the  loads  are  transferred 
through  the  pavement  to  the  road-bed,  and  not  sus- 
tained by  the  pavement  as  a  rigid  structure. 

The  ability  of  earth  to  sustain  a  load  depends  in  a 
large  measure  upon  the  amount  of  moisture  contained 
by  it.  Most  earths  form  a  good  firm  foundation  so 
long  as  they  are  kept  dry,  but  when  wet  they  lose 
their  sustaining  power,  becoming  soft  and  incoherent. 
When  softened  by  moisture  the  soil  may  be  easily 
displaced  by  the  settling  of  the  foundation  of  the 
road,  or  forced  upward  into  any  interstices  that  may 
exist  in  its  superstructure. 

In  cold  climates  the  drainage  of  a  road  is  also  impor- 
tant because  of  the  danger  of  injury  from  freezing. 
Frost  has  no  disturbing  effect  upon  dry  material,  and 
hence  is  an  element  of  danger  only  in  a  road  that 
retains  water. 

26 


DRAINAGE   OF   ROADS   AND    STREETS.  2/ 

In  order,  therefore,  that  the  loads  may  be  uniformly 
sustained,  and  the  surface  of  the  road  kept  firm  and 
even,  it  is  evidently  of  first  importance  that  the  road- 
bed be  maintained  in  a  dry  condition.  The  improve- 
ment and  maintenance  of  a  road  are  therefore  largely 
questions  of  drainage,  the  object  being  to  prevent  water 
from  reaching  the  road  and  to  provide  means  for 
immediately  removing  such  as  does  reach  it  before  the 
soil  becomes  saturated  and  softened. 

Surface  drainage  is  always  necessary  if  the  body  of 
the  road  is  to  be  kept  in  a  dry  condition,  and  is  accom- 
plished by  having  the  surface  of  such  form  that  water 
falling  upon  it  will  quickly  run  into  the  gutters. 

The  necessity  for  underdrainage  in  any  case  depends 
upon  local  conditions,  the  nature  of  the  soil  and  the 
tendency  of  the  site  to  dampness.  Underdrains  are  for 
the  purpose  of  lowering  the  level  of  ground  water  in  wet 
weather  and  preventing  water  from  underground 
sources  reaching  the  road  bed  and  softening  it.  A 
careful  examination  of  local  conditions  is  necessary 
in  any  case  to  determine  the  advisability  of  constructing 
underdrains.  Where  the  soil  upon  which  the  road  is 
constructed  is  so  placed  that  the  ground  water  is  at 
any  time  likely  to  stand  close  to  the  surface  and  become 
soft  immediately  under  the  road-bed,  underdrainage 
is  necessar\r  to  good  results  in  the  maintenance  of  the 
road.  In  any  case  in  which  the  level  of  ground  water 
stands  within  about  3  feet  of  the  surface,  the  road  will 
be  benefited  by  sub-surface  drainage,  although  it  may 
not  be  altogether  necessary  to  the  maintenance  of  the 
road.  Underdrainage  is  of  little  use  for  the  removal 
of  water  from  depressions  in  the  surface  of  the  road. 


28         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  10.     SURFACE  DRAINAGE. 

The  drainage  of  the  surface  of  a  road  is  provided  for 
by  making  the  section  higher  in  the  middle  than  at  the 
sides,  with  ditches  or  gutters  at  the  edges  of  the  road 
along  which  the  water  is  conducted  until  it  may  be  dis- 
posed of  through  some  side  channel. 

The  slope  necessary  from  the  middle  to  the  sides  of 
the  road  to  insure  good  drainage  depends  upon  the 
nature  of  the  road  surface,  being  less  as  the  road  surface 
is  more  smooth  and  less  permeable  to  water.  For 
ordinary  earth  roads  it  varies  from  about  I  in  10  to 
I  in  20;  for  macadam  or  gravel  roads,  from  I  in  15 
to  I  in  30;  and  for  brick  or  asphalt  pavements,  from  I 
in  40  to  I  in  60. 

The  drainage  of  the  surface  of  a  country  road  is' 
mainly  a  matter  of  maintenance,  and  involves  keeping 
the  surface  of  the  road  in  a  smooth  condition  and 
properly  crowned.  It  is  more  fully  discussed  in  Arts. 
25  and  27. 

On  country  roads  the  disposal  of  surface  water  is  not 
usually  a  matter  of  difficulty,  as  it  can  be  carried  along 
the  road  and  run  into  the  first  convenient  cross  channel. 
In  deep  cuts  or  on  steep  grades,  however,  it  may  some- 
times be  economical  to  lay  a  pipe  under  the  gutter 
into  which  surface  water  may  be  turned  at  frequent 
intervals. 

In  all  cases  it  is  important  that  the  water  which 
falls  upon  the  surface  should  be  gotten  rid  of  as  soon 
as  possible,  for  so  long  as  it  remains  upon  the  road  it 
is  an  element  of  danger,  both  from  its  tendency  to 
wash  the  surface,  and  from  its  liability  to  penetrate 
into  the  road  and  thus  cause  disintegration  or  settle- 
ment. 


DRAINAGE  OF  ROADS  AND   STREETS.  2Q 

ART.    II.       SUBDRAINAGE. 

The  drainage  of  the  subsoil  under  a  road  is  intended  to 
lower  the  level  of  ground  water  in  wret  weather  and 
prevent  water  from  sub-surface  sources  reaching  the 
road-bed. 

The  necessity  for  subdrainage,  and  the  method  to 
be  employed  in  any  case,  depends  upon  whether  the 
soil  over  which  the  road  is  being  constructed  is  natu- 
rally wet  or  dry,  and  whether  the  road-bed  is  so  situ- 
ated and  formed  as  to  give  it  natural  drainage. 

The  material  of  which  a  road-bed  is  composed  is 
important  because  it  determines  to  a  large  extent 
whether  artificial  drainage  is  necessary,  and  also  what 
method  should  be  adopted  for  securing  drainage. 

Soils  differ  in  their  power  to  resist  the  percolation 
of  water  through  them,  in  the  rapidity  and  extent  of 
their  absorption  of  water  with  which  they  come  in  con- 
tact, in  the  extent  to  which  moisture  renders  them  soft 
and  unstable,  and  in  their  power  of  retaining  moisture. 

A  light  soil  of  a  sandy  nature  usually  presents  little 
difficulty  in  the  matter  of  drainage,  as,  while  it  is  easily 
penetrated  by  water,  it  is  not  retentive  of  moisture, 
which  passes  freely  through  it  without  saturating  it 
unless  prevented  from  escaping. 

If  the  natural  drainage,  therefore,  have  a  fall  away 
from  a  road-bed  formed  of  such  material,  it  will  usually 
need  no  artificial  drainage,  and  \vhere  subdrains  are  nec- 
essar}^  they  may  be  relied  upon  to  draw  the  water  from 
the  soil  to  a  considerble  distance  each  side  of  the  drain. 

A  nearly  pure  sand  is  more  firm  and  stable,  under 
loads,  wrhen  quite  damp  than  if  dry,  although  a  fine 
sand  saturated  by  water  which  is  unable  to  escape 
may  become  unstable  and  treacherous. 


3O          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Clays  usually  offer  considerable  resistance  to  the 
passing  of  water  through  them,  and  are  very  retentive 
of  moisture.  As  a  rule,  however,  a  clay  soil  does  not 
absorb  water  readily,  and  requires  that  water  be  held 
for  some  time  in  contact  with  it  in  order  that  it  may 
become  saturated,  although  when  saturated  it  is  the 
most  unstable  of  soils.  A  clay  that  when  dry  will 
stand  in  a  vertical  wall  and  support  a  heavy  weight, 
when  wet  may  lose  all  coherence  and  become  a  fluid 
mass.  When  water  comes  in  contact  with  a  bed  of 
such  clay,  the  outside  becomes  saturated  and  semi- 
fluid before  the  moisture  penetrates  into  it  sufficiently 
to  even  moisten  it  a  few  inches  from  the  surface. 

A  clay  soil  is,  therefore,  always  difficult  to  drain  by 
removing  the  water  after  it  has  soaked  in,  or  by  per- 
mitting it  to  pass  through  the  road-bed  to  the  subdrains 
beneath.  Drainage,  in  such  cases,  may  often  be  so 
arranged  as  to  prevent  water  from  standing  against 
the  road  and  thus  prevent  it  from  becoming  saturated. 
As  the  clay  is  comparatively  non-absorptive,  the  water 
which  may  come  upon  its  surface,  if  allowed  to  escape 
at  once,  will  not  penetrate  into  it,  and  hence  will  not 
cause  softening. 

A  heavy  silt  formation  is  sometimes  met  with  which 
is  even  more  difficult  to  drain  than  a  true  clay.  It  is 
nearly  as  retentive  of  moisture  as  a  clay,  strongly 
resisting  the  passage  of  water  through  it,  but  at  the 
same  time  absorbs  water  quite  freely  when  in  contact 
with  it. 

Between  the  extremes  mentioned  above  there  are 
a  great  number  of  varieties  of  soil  which  possess  to  a 
greater  or  less  extent  the  characteristics  of  either  or 
both,  and  gradually  merge  the  one  into  the  other.  In 
applying  a  system  of  drainage  in  any  case,  careful 


DRAINAGE  OF  ROADS  AND   STREETS.  31 

attention  should  always  be  given  to  the  characteristics 
of  the  soil,  as  determining  very  largely  the  treatment 
to  be  used. 

Where  artificial  subdrainage  is  necessary  the  drains 
should  be  located,  in  so  far  as  possible,  with  a  view  to 
cutting  off  the  supply  of  water  before  it  reaches  the 
road-bed.  To  accomplish  this  to  the  best  advantage 
the  local  conditions  must  be  observed,  the  sources  of 
this  supply  determined,  and  the  nature  of  the  under- 
flow, if  any  exist,  considered.  In  most  instances  on 
roads  over  soil  commonly  met  upon  country  roads  a 
single  line  of  tile  under  one  side  of  the  road  will  lower 
the  ground  water  sufficiently  to  prevent  it  reaching 
the  road-bed. 

Frequently,  as-  in  many  cases  of  a  road  along  a  side 
slope,  there  is  a  well-defined  flow  of  sub-surface  water 
from  one  side  to  the  other,  and  in  such  case  the  water 
may  perhaps  be  intercepted  by  a  single  longitudinal 
drain  on  the  side  of  the  roadway  from  which  the 
water  comes.  An  example  of  this  is  shown  in  Fig.  3. 


FIG.  3. 

When  the  subsoil  is  of  stiff  and  retentive  material 
which  does  not  drain  readily,  an  underdrain  on  one  side 
may  not  draw  the  water  from  under  the  wrhole  width 
of  the  roadway.  In  this  case  it  is  advisable  to  use  a 
drain  on  each  side  to  cut  ofT  the  water  before  it  reaches 
the  roadway.  This  may  be  necessary  with  a  clay  soil 
when  the  line  of  ground  water  is  high. 

Sometimes  a  single  drain  is  laid  under  the  middle  of 


32         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  road,  as  shown  in  Fig.  4.  This  is,  in  general,  an 
undesirable  practice;  the  middle  of  the  roadway  is 
not  a  convenient  place  for  the  drain,  and  necessitates 
digging  a  trench  under  the  roadway  which  is  likely 
to  give  considerable  trouble  in  the  early  maintenance 
of  the  road  surface.  In  some  instances  however  a 
spring  of  water  may  come  up  under  the  roadway,  as  in 
a  clay  spout,  and  when  this  occurs  it  is  desirable  to  lay 
a  pipe  to  take  the  water  from  the  source  of  supply 
rather  than  to  drain  it  through  the  soil  to  the  side 
drains. 

The  most  satisfactory  and  cheapest  method  of  under- 


FIG.  4. 

drainage  is  commonly  by  the  use  of  porous  drain  tile, 
as  used  for  farm  drainage.  Where  stone  is  plentiful 
and  handy  to  the  road,  a  stone  drain  may  be  cheap  and 
equally  effective  with  the  tile.  These  types  of  drains 
are  described  in  succeeding  articles. 

Many  road  builders  utilize  the  side  ditches,  intended 
for  surface  drainage,  for  underdrainage  also  by  making 
them  deep  and  narrow.  This  is  not  usually  an  eco- 
nomical practice.  A  tile  drain  and  shallow  gutters 
will  not  be  more  expensive  to  construct  than  the  deep 
ditches,  while  they  are  much  easier  and  cheaper  to 
maintain.  In  some  instances  tiles  are  laid  under  the 
surface  ditches  and  the  trenches  filled  with  stones,  or 
gravel,  as  shown  in  Fig.  5,  thus  permitting  the  surface 
drainage  to  seep  into  the  tile.  This  gives  very  effective 


DRAINAGE   OF  ROADS  AND   STREETS.  33 

drainage,  if  the  tile  be  of  sufficient  capacity,  but  is 
expensive  to  construct. 

In  considering  the  advisability  of  underdrainage  and 
the  method  of  accomplishing  it,  the  fact  should  be 
kept  in  mind  that  the  purpose  of  underdrainage  is  to 
remove  ground  water,  and  that  efficient  drainage  of 


FIG.  5. 

the  road  surface  can  only  be  accomplished  by  main- 
taining the  surface  in  smooth  condition  and  of  proper 
form. 

ART.  12.     TILE  DRAINS. 

Tile  drains  for  road  drainage  are  constructed  in  the 
same  manner  as  for  land  drainage.  Ordinary  porous 
tiles  are  used  as  in  farm  drainage,  sizes  from  4  to  8  or 
10  inches  in  diameter  being  commonly  employed  for 
this  purpose.  They  are  usually  in  lengths  of  slightly 
more  than  12  inches,  the  excess  of  length  being  sufficient 
to  allow  for  probable  breakage,  so  that  estimates  may 
be  made  on  the  basis  of  one  tile  to  each  foot  of  length. 
The  tiles  should  be  truly  cylindrical  with  the  ends  cut 
off  square,  and  be  smooth  inside.  They  are  laid  in  a 
trench  3  or  4  feet  below  the  surface  of  the  ground,  with 
their  ends  in  contact.  They  should  be  carefully  placed 
so  that  the  ends  fit  closely,  and  the  bottom  of  the 
trench  should  be  cut  to  about  the  width  of  the  tile,  so 
that  they  cannot  move  sideways  when  the  trench  is 


34          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

filled;  or  better  still,  a  groove  may  be  scooped  out  in 
the  bottom  of  the  trench  to  fit  the  tile. 

Grade  of  Tile.  The  velocity  of  water  through  a  tile 
depends  upon  the  slope  of  the  tile.  Considerable  water 
may  be  carried  by  a  tile  laid  to  an  almost  level  grade. 
Such  grades  are,  however,  rarely  necessary  in  road 
drainage  and  are  to  be  avoided  whenever  possible.  It 
is  not  desirable,  except  under  unusual  conditions,  to 
use  a  grade  less  than  about  2  inches  per  100  feet.  This 
gives  a  velocity  which  may  be  reasonably  expected  to 
keep  the  tile  clear,  when  properly  laid,  except  for  small 
tiles,  although  a  greater  slope  is  to  be  preferred  when 
obtainable. 

Care  should  be  used  in  laying  tile  to  place  it  accu- 
rately to  the  grade  line,  particularly  when  the  slope  is 
light.  Irregularities  are  apt  to  produce  depressions  in 
which  deposit  of  silt  may  take  place. 

Size  of  Tile.  A  tile  for  road  drainage  should  not  be 
less  than  4  inches  in  diameter.  While  a  smaller  tile 
may  often  be  large  enough  to  carry  the  water,  the 
danger  of  clogging  is  much  greater  and  the  effect  of 
inequalities  in  grade  are  increased  for  such  tile.  The 
size  of  tile  required  depends  upon  the  quantity  of  water 
to  be  carried  and  the  slope  of  the  tile.  For  agricultural 
drainage  it  is  common  to  assume  that  the  tile  must 
remove  from  J  inch  to  I  inch  of  water  per  day  over  the 
area  which  it  drains.  The  rules  commonly  followed 
probably  give  an  excessive  run-off  in  most  instances, 
and  recent  observations  indicate  that  J  inch  would  be 
ample  in  most  instances  of  ordinary  drainage.  This 
method  may  be  applied  in  road  work  where  the  area 
from  which  the  water  is  drawn  can  be  determined. 
The  area  to  be  included  depends  upon  the  character 
of  the  soil  and  the  way  the  ground  lies.  On  level 


DRAINAGE   OF  ROADS   AND    STREETS.  35 

ground  the  drain  may  be  assumed  to  receive  water 
from  a  certain  distance  on  each  side  depending  upon 
the  porosity  of  the  soil. 

For  road  drainage  the  size  of  tile  used  should  be 
such  as  to  provide  liberal  capacity.  Comparatively 
small  sizes  will  usually  be  required  and  the  differ- 
ences in  cost  are  small.  An  area  of  25  to  50  feet  on 
each  side  may  be  considered  as  contributing  water 
to  the  tile.  In  ordinan^  soil  the  effect  of  the  tile  will 
reach  much  farther  than  this,  but  the  percolation  is 
so  slow  that  the  water  will  reach  the  tile  very 
gradually. 

This  method  may  serve  as  a  guide  in  selecting  the 
size  of  tile  required,  but  is  not  capable  of  accurate 
computation  and  is  onl3T  of  value  as  an  aid  to  judg- 
ment. Good  practice  in  such  work  must  rest  mainly 
upon  the  judgment  derived  from  experience.  If  the 
tile  be  supposed  to  collect  water  from  about  25  feet 
on  each  side,  it  would  drain  about  an  acre  for  each 
870  feet  of  length,  or  about  6  acres  per  mile.  Assum- 
ing one-half  inch  in  24  hours,  over  the  drainage  area, 
as  the  amount  to  be  provided  for,  one  acre  will  yield 
1815  cubic  feet  per  day,  or  ij  cubic  feet  per  minute; 
and  one  mile  of  length,  50  feet  wide,  will  yield  about 
7?  cubic  feet  per  minute. 

The  water  canying  capacity  of  tile  drains  has  not 
been  accurately  determined  but  it  probably  does  not 
greatlv  differ  from  that  for  vitrified  pipe  sewers,  and 
the  use  of  the  formulas  usuall}T  applied  to  sewers  will 
be  sufficiently  accurate  for  practical  purposes.  The 
common  formula  for  the  flow  of  water,  v  =  C  ^/RS,  may 
for  our  purpose  be  transposed  into  the  form  V  =  k  ^/DS, 
in  which  V  is  the  velocity  in  cubic  feet  per  second, 
D  is  the  diameter  of  the  pipe  in  inches,  S  is  the  slope, 


A  TEXT-BOOK:  ON  ROADS  AND  PAVEMENTS. 


and  k  is  a  coefficient  varying  from  about  9  for  4-inch 
pipe  to  12  for  12-inch  pipe. 

The  following  table  for  the  capacity  of  tile  drains  is 
based  upon  this  formula.  It  is  computed  by  the  use 
of  Kutter's  formula,  using  a  coefficient  of  roughness 
of  .013,  which  corresponds  to  the  flow  in  pipe  sewers. 

CAPACITY    OF    TILE    DRAINS    IN    CUBIC    FEET    PER    MINUTE 


Slope  per  too 
Feet. 

Sizes  of  Pipe. 

In. 

Feet. 

4  In. 

6  In. 

8  In. 

10  In. 

i  2  In. 

2  . 

.67 

4- 

12  . 

27. 

49-5 

81. 

4- 

•33 

5  -5 

i6.5 

38. 

70. 

114. 

6. 

•5° 

6-5 

21  . 

46.5 

86.5 

143- 

9- 

•75 

8. 

25-5 

57-5 

106  .5 

176. 

12  . 

i  . 

9-5 

29-5 

66. 

122.5 

204  . 

24. 

2  . 

13-5 

41-5 

92-5 

173- 

288. 

36. 

3- 

16.5 

Si- 

114  . 

212  . 

353- 

48. 

4- 

19. 

59- 

132. 

245- 

408. 

60. 

5- 

21  . 

66. 

148. 

275- 

456. 

Tiles  laid  upon  very  flat  slopes  sometimes  may  carry 
a  quantity  of  water  greater  than  the  capacity  due  to 
the  slope;  this  is  caused  by  the  level  of  ground  water 
standing  above  the  tile,  thus  causing  the  water  to 
flow  in  the  tile  under  a  head  greater  than  that  due 
to  its  slope.  Where  gravel  or  other  porous  material 
is  available  such  tile  will  be  benefited  by  a  porous 
filling  immediately  over  the  tile.  This  also  assists  in 
keeping  the  tile  clear  of  sediment. 

ART.  13.     STONE  DRAINS. 

In  localities  where  stone  suitable  for  such  purposes 
exists  along  a  roadway  it  is  common  and  often  econ- 
omical to  use  stone  drains  for  purposes  of  under- 
drainage. 


DRAINAGE   OF  ROADS   AND    STREETS. 


37 


Blind  Drains.  For  short  lengths,  where  it  is  only 
necessary  to  provide  a  permeable  path  for  a  small 
quantity  of  water  to  escape,  blind  drains  may  be  used. 
They  consist  of  ditches  cut  into  the  soil  and  filled  at 
bottom  with  fragments  of  stone,  the  trench  then 
filled  with  earth.  Care  should  be  taken  that  the  top 
of  the  stone  is  protected,  so  that  the  earth  may  not 
be  washed  into  the  stone  and  stop  the  drain;  a  little 
small-sized  stone  or  gravel  on  top,  or  a  light  layer  of 
brush  or  sod,  to  hold  the  earth  until  it  has  compacted, 
is  useful.  Such  drains  have  frequently  proven  quite 
efficient  when  used  where  the  requirements  are  not  too 
great. 

Box  Drains.  Where  suitable  stone  is  plenty  and 
cheap,  a  box  drain  may  be  built.  This  consists  of  a 


FIG.  6. 


FIG.  7. 


rectangular  box  formed  of  flat  stones  at  the  bottom  of 
the  trench,  which  is  then  filled  with  earth.  This  box 
may  be  very  roughly  built,  and  it  is  desirable  when 
stone  or  gravel  is  plentiful  to  fill  immediately  over 
the  drain  with  such  material,  to  protect  it  against  the 
entrance  of  earth  and  assist  in  leading  the  water  into 
it.  Figure  6  shows  a  section  of  such  a  drain  as  con- 


38          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

structed  to  intercept  a  seepage  of  water  in  stiff  reten- 
tive material.  In  ordinary  soil  there  would  be  no 
advantage  in  filling  the  trench  so  full  of  stone.  The 
construction  shown  in  Fig.  7  is  also  sometimes  used, 
and  is  cheap  and  reasonably  efficient. 

The  size  of  opening  in  a  stone  drain  must  be  consid- 
erably larger  than  that  in  a  tile  to  carry  the  same 
quantity  of  water,  the  construction  usually  being  very 
rough,  and  the  resistance  to  flow  greater.  Drains  of 
this  type  are  used  in  many  localities  where  materials 
are  available  for  building  them,  although  their  use  is 
growing  less,  due  to  the  fact  that  porous  tile  costs  so 
little  and  tile  drains  are  so  easily  and  cheaply  con- 
structed. 

ART.  14.     CULVERTS. 

Culverts  are  commonly  required  in  road  construction 
for  carrying  under  the  road  the  small  streams  which 
may  be  crossed  by  the  road,  or  sometimes  for  carry- 
ing the  water  collected  in  the  gutters  or  ditches  on 
the  upper  side  of  the  road  to  the  lower  side. 

The  waterway  provided  by  a  culvert  must,  for 
safety,  be  sufficiently  large  to  pass  the  maximum  flow 
of  water  that  is  likely  to  occur,  while  for  economy  it 
must  be  made  as  small  as  may  be  without  danger. 

The  maximum  flow  of  a  stream  depends  upon  a 
number  of  local  conditions,  most  of  which  are  very 
difficult  of  accurate  determination.  These  are:  the 
maximum  rate  of  rainfall,  the  area  drained  by  the 
stream  and  its  position,  the  character  of  the  surface 
drained,  and  the  nature  of  the  channel. 

The  maximum  rate  of  rainfall  varies  in  different 
localities,  and  differs  in  the  same  locality  from  year  to 
year.  It  is  commonly  taken  at  about  an  inch  an  hour. 


DRAINAGE   OF  ROADS   AND   STREETS.  39 

This  is  sometimes  exceeded  for  a  very  short  time  and 
over  a  small  area,  but  is  usually  a  safe  value  for  a 
watershed  of  any  considerable  area. 

The  approximate  area  of  the  watershed  drained  by 
a  stream  is  readily  found,  and  its  form  is  also  impor- 
tant as  determining  the  distance  the  water  must  flow  in 
reaching  the  culvert  under  consideration,  and  to  some 
extent  regulating  the  rate  at  which  the  water  falling 
upon  the  area  will  reach  the  culvert. 

The  maximum  flow  of  a  stream  is  also  affected  by 
the  physical  characteristics  of  the  watershed.  The 
permeability  of  the  surface  largely  determines  what 
portion  of  the  rainfall  shall  reach  the  stream;  while  the 
slope  of  the  surface,  its  evenness,  and  its  vegetation 
have  an  effect  upon  the  quickness  and  rate  with  which 
the  rainfall  is  received  by  the  stream. 

The  determination  of  the  maximum  flow  to  be  ex- 
pected in  any  case  from  an  examination  of  the  locality 
is  therefore  possible  only  as  a  very  rough  approxima- 
tion. A  number  of  formulae  have  been  proposed  for 
such  estimation,  the  use  of  which  for  the  case  of  an 
ordinary  culvert  simply  amounts  to  estimating  the 
quantity  of  water  which  would  fall  on  the  watershed  in 
the  heaviest  probable  rain,  and  judging  as  well  as  pos- 
sible from  local  conditions  ho\v  much  of  it  may  arrive 
at  one  time  at  the  culvert.  In  some  cases  where  a 
more  accurate  determination  is  desirable  it  may  be 
advisable  to  measure  the  flow  of  the  stream  at  high 
water,  and  form  an  idea  from  such  measurement  as  to 
what  may  be  expected  at  a  maximum  stage. 

The  amount  of  water  that  will  pass  a  culvert  in  a 
given  time  depends  upon  the  form  of  the  section,  the 
smoothness  of  its  interior  surface,  its  slope,  and  the 
head  under  which  the  water  is  forced  through.  A 


4O         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

well-constructed  culvert  may  be  considered  in  comput- 
ing its  capacity  as  a  pipe  flowing  full.  Other  culverts 
or  bridges  must  be  treated  as  open  channels. 

Prof.  Talbot  gives  (Selected  Papers  C.  E.  Club,  Univ. 
of  Illinois,  1887-8)  a  formula  for  the  rough  determina- 
tion of  area  required  for  waterway,  derived  from  ex- 
perience : 
Area  waterway  in  feet  =  C  V  (drainage  area  in  acres)  .3 

C  is  a  coefficient  depending  upon  local  conditions. 
For  rolling  agricultural  country  subject  to  floods  at 
time  of  melting  snow,  and  with  length  of  valley  3  or  4 
times  the  width,  C  =  J.  When  the  valley  is  longer, 
decrease  C.  If  not  affected  by  snow  and  with  greater 
lengths,  C  may  be  taken  at  i,  J,  or  even  less.  For 
steep  side  slopes  C  should  be  increased. 

For  most  cases  in  practice  the  size  of  waterway 
required  may  be  determined  from  the  knowledge  which 
usually  exists  in  the  vicinity  regarding  the  character  of 
a  stream,  from  the  sizes  of  other  openings  upon  the 
same  stream,  or  from  comparison  with  other  streams 
of  like  character  and  extent  in  the  same  locality. 
Where  data  of  this  kind  do  not  exist,  careful  exami- 
nation of  water-marks  on  rocks,  the  presence  of  drift, 
etc.,  may  be  made  to  determine  the  height  to  which 
water  has  previously  risen.  The  shape  of  the  valley 
and  the  slope  of  the  surface  is  of  more  importance 
than  the  area  of  country  drained.  The  use  of  a  for- 
mula like  Talbot's  assists  the  arrangement  of  the 
factors  which  enter  into  the  determination,  and  is 
only  intended  as  an  aid  to  judgment  in  selecting  the 
size  of  opening  required. 

The  discharging  capacity  of  a  culvert  will  depend 
upon  the  slope  of  the  water  surface  passing  through 
the  culvert.  Increasing  the  slope  of  the  bed  of  the 


DRAINAGE   OF  ROADS   AND    STREETS.  4! 

culvert  will  increase  its  carrying  capacity,  provided 
the  water  can  flow  freely  away  below  the  culvert.  If 
a  culvert  be  so  constructed  as  to  permit  the  water 
to  dam  up  above  it,  causing  the  culvert  to  flow  full  and 
under  pressure,  the  effect  is  the  same  as  increasing  the 
slope  and  increases  the  capacity  of  the  culvert.  The 
velocity  through  a  culvert  is  approximately  propor- 
tional to  the  square  root  of  the  head  of  water,  the 
head  being  the  difference  of  elevation  of  the  water 
surface  at  the  entrance  to  the  culvert  and  that  where 
the  water  leaves  the  culvert. 

There  are  three  types  of  culverts  in  common  use  for 
road  purposes:  stone  box  culverts,  pipe  culverts,  and 
concrete  culverts.  In  some  localities  wooden  box  cul- 
verts are  also  used  for  this  purpose;  these  are  very  un- 
economical on  account  of  their  perishable  nature  and 
their  use  should  be  abandoned. 

Stone  Culverts.  Culverts  of  stone  may  be  either 
arch  culverts  or  box  culverts.  Box  culverts  are  usualty 
formed  of  two  side  walls  and  a  cover.  The  side  walls 
consist  usually  of  rubble  stonework  laid  up  dry  or  in 
mortar,  as  the  case  may  be.  Where  the  stream  to  be 
carried  is  of  small  importance,  and  the  capacity  of  the 
culvert  not  greatly  taxed,  dry  walls  ma^^  give  satis- 
factory results,  but  when  the  culvert  is  likely-  to  flow 
full  at  certain  times  it  should  be  laid  up  in  hydraulic 
cement  mortar,  and  in  any  case  the  greater  stability 
given  by  the  mortar  would  be  well  worth  the  small 
additional  cost.  Fig.  8  shows  a  section  of  the  ordi- 
nary form  of  box  culvert.  The  use  of  head  walls  and 
paving  the  waterway  for  a  short  distance  is  necessarj^ 
for  these,  as  for  pipe  culverts. 

Where  suitable  stone  is  available,  box  culverts  are 
easily  constructed  and  economical.  They  are  com- 


A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 


monly  used  for  openings  2  to  4  feet  in  width  and 
2  to  5  feet  in  height.  The  width  that  may  be  used 
depends  upon  the  available  cover  stones.  Where  the 
allowable  width  is  not  sufficient  to  give  the  needed 
area  of  waterway,  a  double  culvert  may  sometimes  be 
used  to  advantage.  This  consists  of  two  openings 
with  a  middle  wall  to  support  the  covers. 

The  culvert's  opening  should  always  be  large  enough 
to  admit  of  a  man  passing  through  it  for  the   purpose 


V/7////A 


of  cleaning  it  —  at  least  18  by  24  inches.  The  side  walls 
should  extend  downward  below  the  bottom  of  the 
culvert  sufficiently  to  obtain  a  good  foundation,  and 
the  thickness  required  for  the  side  walls  usually  varies 
from  one-half  to  three-fourths  the  height,  depending 
upon  the  pressure  likely  to  come  against  them. 

In  many  cases  for  small  work  the  side  walls,  instead 
of  extending  downward,  rest  upon  the  paving  which  is 
extended  under  them.  This  gives  a  somewhat  less 
expensive  construction,  and  is  often  satisfactory  on 
good  ground. 

The  cover  stones  may  be  from  \  to  J  the  span  in 
thickness,  and  should  be  long  enough  to  have  a  bear- 


DRAINAGE   OF  ROADS   AND    STREETS.  43 

ing  upon  each  side  wall  of  at  least  one-half  the  thick- 
ness of  the  wall. 

Pipe  Culverts.  Pipe  culverts  may  be  constructed 
either  of  salt-glazed  vitrified  sewer  pipe,  or  of  iron 
water  pipe.  For  culverts  of  sizes  up  to  about  30  inches 
diameter,  vitrified  pipe  is  often  the  most  economical 
material  to  use  provided  it  is  placed  on  good  founda- 
tion and  sufficiently  covered  not  to  be  subject  to  shock 
from  the  traffic.  The  iron  pipe  possesses  greater 
strength,  and  is  preferable  where  a  firm  foundation  is 
not  easily  obtained,  or  where  a  sufficient  covering  can 
not  be  had  for  the  vitrified  pipe,  as  it  is  not  so  easily 
broken  by  a  slight  settlement  or  by  shocks.  It  is 
somewhat  expensive  and  not  economical  for  ordinary 
use. 

In  laying  pipe  culverts,  they  should  be  placed  on  a 
solid  bed,  and  the  earth  be  well  tamped  about  them. 
It  is  desirable  to  have  the  bottom  of  the  trench  exca- 
vated to  fit  the  lower  part  of  the  pipe,  depressions 
being  formed  for  the  sockets.  It  is  necessary  in  every 
case  that  the  pipe  be  firmly  and  uniformly  supported 
from  below,  in  order  that  the  culvert  may  not  be 
broken  by  settlement,  which  is  especially  likely  to 
occur  in  new  work. 

The  joints  in  the  pipe  should  be  made  water-tight, 
especially  where  the  culvert  is  likely  to  flow  full  or 
under  pressure,  as  any  water  escaping  through  the 
joints  will  tend  to  cause  a  wash  beneath  the  pipe  and 
undermine  the  culvert.  Joints  are  commonly  filled 
with  clay,  but  where  strength  is  needed  the  use  of 
hydraulic  cement  mortar  is  preferable.  The  cost  of 
filling  joints  is  small  and  adds  much  to  the  security 
of  the  culvert. 

Care  should  be  taken  that  the  culvert  have  sufficient 


44 


A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 


slope  and  be  so  placed  that  water  may  not  stand  in 
it,  in  order  to  prevent  injury  from  freezing.  When  this 
is  not  feasible,  iron  pipe  should  be  used.  The  top  of 
the  culvert  pipe  should  be  at  least  1 8  inches  below  the 
road  surface  to  avoid  crushing,  and  for  the  larger  sizes 
of  pipe  (24  to  36  inches),  at  least  two  feet. 

The  ends  of  pipe  culverts  should  be  set  in  masonry 
walls  to  give  protection  against  the  washing  of  the 
face  of  the  embankment,  hold  the  ends  firmly  in  place, 
and  prevent  the  entrance  of  water  into  the  earth  on 
the  outside  of  the  pipe. 

These  walls  to  give  efficient  protection  must  be  of 
substantial  construction,  going  down  to  a  solid  founda- 
tion below  the  bed  of  the  stream.  They  may  be  built 
of  rubble  masonry,  and  should  be  laid  up  in  hydraulic 
cement  mortar.  Such  construction  is  represented  in 
Fig.  9.  The  wall  must  extend  far  enough  on  the  side 


FIG.  9. 

to  sustain  the  earth  of  the  embankment  from  the 
waterway,  or  wing  walls  may  be  used  extending  up 
stream  for  this  purpose.  The  \vaterway  should  be 
paved  above  the  culvert  far  enough  to  prevent  scour- 
ing at  the  base  of  the  wall. 


DRAINAGE   OF  ROADS   AND    STREETS.  45 

For  quite  small  streams  the  walls  may  sometimes 
be  omitted  if  the  face  of  the  embankment  about  the 
entrance  to  the  pipe  and  the  waterway  for  some  dis- 
tance above  and  below  be  riprapped.  Where  it  is 
necessary  to  economize  in  the  cost  of  construction, 
this  method  is  preferable  to  the  use  of  very  light  end 
walls. 

On  streams  too  large  for  a  single  pipe  it  is  often 
economical  to  lay  two  or  three  pipes  side  by  side, 
rather  than  to  construct  an  arch  or  the  open  way  of  a 
bridge.  In  laying  large  pipes  it  is  usually  advisable 
to  place  a  broken-stone  or  concrete  foundation  under 
the  pipes  throughout  their  lengths  to  insure  uniform 
support. 

ART.  15.     CONCRETE  CULVERTS. 

Where  the  waterway  required  is  too  large  to  permit 
the  use  of  vitrified  pipe,  concrete  culverts  are,  in  most 
instances,  the  most  economical  to  use,  and  in  many 
locations  they  may  be  placed  more  cheaply  than  the 
larger  sizes  of  pipe  culverts.  Concrete,  made  of  good 
materials,  and  properly  mixed  and  placed,  is  a  very 
durable  material  and  will  last  indefinitely.  A  well 
designed  concrete  culvert  should  therefore  require  very 
little  in  the  way  of  maintenance. 

These  culverts  are  built  either  with  arched  or  flat 
tops.  For  small  spans,  the  rectangular  box  form  is 
usually  the  most  economical.  The  arched  culvert  for 
small  spans  is  usually  built  of  solid  concrete  without 
reinforcement,  and  is  heavier  than  the  box  form,  unless 
the  culvert  be  very  small.  For  longer  spans  the  rein- 
forced arch  is  desirable. 

Fig.  10  shows  the  section  of  a  concrete  culvert  in 
which  the  sides  and  bottom,  as  well  as  the  top,  are 


46       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS 


reinforced  with  steel  rods,  for  the  purpose  of  taking 
the  tension  due  to  the  tendency  to  bend  under  the  loads 
which  come  upon  it.  Concrete  is  a  good  material  for 
resistance  to  compression,  but  offers  slight  resistance 
to  tension;  the  introduction  of  steel  rods  to  take  the 
tensions,  therefore,  make  it  possible  to  construct  the 
walls  and  top  of  the  culvert  much  lighter  than  they 
could  otherwise  be  built. 

Structures  of  this  kind  must  be  carefully  designed 
and  constructed  in  order  to  secure  good  results.  The 
steel  should  consist  of  small  rods  well  bedded  in  the 
concrete.  They  should  be  placed  with  the  center  of 

the  rods  about  two 
inches  from  the  inner 
surf  ace  of  the  concrete. 
The  area  of  steel  re- 
quired in  the  top  of 
such  a  culvert  is  usu- 
ally about  one  per 
cent  of  the  area  of  the 
concrete  above  it. 
Concrete  culverts, 


Fig-10  like  pipeculverts, must 

be  protected  by  a  covering  of  earth  from  the  shocks 
of  the  traffic.  This  covering  should  be  1 8  inches  in 
thickness,  and  in  no  case  should  be  less  than  12  inches. 
The  ends  of  the  culvert  must  be  protected  by  walls, 
which  should  extend  at  least  two  feet  below  the  bottom 
of  the  culvert. 

The  thickness  of  the  top  and  sides  must  depend  upon 
the  loads  which  may  come  upon  the  culvert  and  upon 
the  character  of  the  concrete.  They  should  usually 
be  designed  to  safely  carry  a  heavy  road  roller.  The 
concrete  should  be  made  of  the  best  grade  of  Portland 


DRAINAGE   OF   ROADS   AND    STREETS. 


47 


cement  mixed  with  good  qualit\-  sand  and  gravel,  or 
broken  stone,  so  as  to  produce  a  very  dense,  homo- 
geneous concrete,  the  proportions  for  the  top  being 
about  I  part  cement,  2  parts  sand,  4  parts  broken 
stone;  for  the  sides  and  bottom,  i  part  cement,  2\ 
parts  sand,  5  parts  broken  stone  or  I  part  cement,  3 
parts  sand,  6  parts  broken  stone.  The  following  tables 
give  approximate  dimensions  for  culverts  suitable  for 
country  roads  under  these  conditions: 


TOP    OF    CULVERT. 

Span  in  feet 

•^ 

A 

6 

8 

Thickness  of  concrete,  —  inches.  . 

8 

4- 

9 

^> 

10 

ir 

13 

Size  of  steel  bars,  —  inches  square 

i 

f 

i 

i 

1 

Distance  apart  of  bars,  c.  to  c.  — 

inches 

6 

8 

7 

6 

6 

BOTTOM    OF    CULVERT. 

/ 

Span  in  feet  

3 

4 

5 

6 

8 

Thickness  of  concrete,  —  inches.  .  . 

5 

6 

7 

8 

9 

Size  of  steel  bars,  —  inches  square 

I 

$ 

i 

1 

i 

Distance  apart  of  bars,  c.  to  c.  — 

inches  

18 

12 

10 

16 

12 

SIDES    OF    CULVERT. 

Height  of  opening,  —  feet  

2 

3 

4 

5 

6 

Thickness  of  concrete,  —  inches. 

6 

6 

7 

8 

9 

Size  of  steel  bars,  —  inches  square  . 

\ 

i 

i 

I 

1 

Distance  apart  of  bars,  c.  to  c.  — 

inches  .  . 

12 

10 

12 

10 

12 

It  may  sometimes  be  desirable  to  leave  out  the  con- 
crete bottom,  and  extend  the  side  \valls  deeper,  as  with 
a  stone  box  culvert.  Where  this  is  done,  the  side  walls 
should  extend  at  least  1 8  inches  below  the  bottom  of 
the  culvert,  and  should  widen  at  the  bottom  into  a 
footing  which  will  give  a  firm  foundation  to  the 
structure. 


48         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

For  small  culverts  on  country  roads  the  side  walls 
may  be  of  plain  concrete,  with  a  thickness  of  about 
one-third  of  the  height.  For  the  smaller  sizes  this 
may  in  many  instances  be  cheaper  than  the  reinforced 
sides.  For  openings  not  more  than  1 8  inches  to  2  feet 
square  a  semicircular  arched  opening,  without  rein- 
forcement, with  a  thickness  of  arch  and  side  wall  of 
about  one-third  the  diameter,  may  be  cheaper  than  the 
rectangular  form  of  opening. 

In  work  of  this  kind,  great  care  must  be  taken  to 
secure  good  materials;  the  broken  stone  should  be  of 
good  hard  material,  not  too  uniform  in  size,  varying 
from  about  f-inch  to  ij  or  ij  inches;  sand  should 
preferably  be  coarse  and  not  uniform  in  size,  it  should 
be  clean,  hard  sand;  cement  should  meet  the-  specifi- 
cations of  the  American  Society  for  Testing  Materials 
for  Portland  cement.  The  mixing  and  placing  of  the 
concrete  must  be  carefully  done  so  as  to  secure  a 
thorough  and  uniform  mixture  of  the  ingredients,  and 
a  dense,  compact  mass  of  concrete  in  the  culvert. 


CHAPTER    III. 

LOCATION  OF  COUNTRY  ROADS. 
ART.    1 6.     CONSIDERATIONS  GOVERNING  LOCATION. 

THE  determination  of  a  line  for  a  proposed  road 
involves  the  examination  of  the  country  through  which 
the  road  is  to  pass  with  reference  to  its  topographical 
features,  the  nature  and  extent  of  the  traffic  that  it 
may  develop,  and  the  local  interests  that  may  be 
affected  by  the  position  of  the  road. 

The  simplest  form  that  this  problem  can  take  is  that 
in  which  two  points,  as  two  towns,  are  to  be  connected 
by  a  road  for  the  purpose  of  providing  for  a  traffic 
between  them,  the  nature  and  amount  of  which  is 
approximately  known.  In  this  case  it  is  only  necessary 
to  examine  the  topography  of  the  intervening  country 
and  select  the  line  over  which,  taking  into  account 
the  costs  of  construction  and  maintenance,  the  given 
traffic  may  be  most  economically  carried. 

In  most  cases  in  practice,  however,  the  problem 
does  not  have  this  simple  character,  and  in  fact  location 
can  seldom  be  determined  by  considerations  of  economy 
alone.  The  position  of  the  line  will  be  modified  by 
local  needs,  such  as  the  necessity  of  providing  for  the 
traffic  of  villages  or  farms  intermediate  between  the 
ends  of  the  road,  which  may  often  cause  deviations 
from  what  would  be  the  best  line  if  the  interests  of 
the  terminal  points  alone  were  considered. 

Questions  of  the  desirability  of  various  lines  for  the 
comfort  and  convenience  of  travel,  and  the  pleasure  to 

49 


50         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

be  derived  from  the  use  of  the  road,  dependent  upon 
aesthetic  considerations,  may  also  frequently  operate 
to  change  the  line  from  what  would  seem  proper  from 
a  strictly  economic  point  of  view. 

In  thickly  settled  communities,  as  in  most  parts  of  the 
United  States,  the  roads  are  in  the  main  already  located, 
the  necessity  for  the  location  of  new  ones  does  not  often 
arise,  and  when  it  does  occur  is  usually  mainly  deter- 
mined by  the  local  needs  and  requirements  of  traffic. 

The  economic  considerations  involved  in  the  location 
of  roads  are  of  two  kinds:  those  relating  to  the  accom- 
modation of  traffic,  and  those  relating  to  its  economic 
conduct.  The  first  deals  with  the  necessity  of  the  road 
to  the  community,  the  second  with  the  cost  of  operat- 
ing it.  The  first  involves  the  general  question  of  the 
advisability  of  any  road,  and  how  it  can  be  placed  to 
give  the  greatest  freedom  to  the  movement  of  travel. 
The  question  is  as  to  the  value  of  the  road  to  the  gen- 
eral community  and  its  location  to  secure  the  greatest 
good  for  the  least  outlay,  without  taking  into  account 
the  details  of  location  which  may  affect  the  cost  of 
transportation.  The  value  of  the  road  as  developing 
trade  in  a  town  or  bringing  a  farm  nearer  to  market 
would  enter  into  consideration.  The  accommodation 
of  traffic  requires  that  a  road  be  located  with  a  view  to 
the  convenience  of  its  use  by  the  largest  portion  of  the 
traffic,  as  well  as  with  a  view  of  developing  traffic. 

The  position  of  a  road  that  will  best  accommodate 
traffic  is  that  in  which,  other  things  being  equal,  the 
mass  of  traffic  need  be  moved  the  least  distance  in 
reaching  its  destination;  or,  in  other  words,  that  for 
which  if  each  ton  of  freight  be  multiplied  by  the  dis- 
tance through  which  it  must  be  moved  the  summation 
of  the  resulting  products  will  be  a  minimum.  If  there 


LOCATION  OF  COUNTRY  ROADS.        51 

be  differences  in  the  nature  of  the  routes  over  which 
the  road  may  be  constructed,  they  may  be  considered 
as  equivalent  to  changes  in  the  relative  effective  lengths 
of  line  for  purposes  of  comparison. 

The  ordinary  problem  of  location  deals  mainly  with 
considerations  of  the  second  class.  It  consists  for  the 
most  part  in  the  relocation  of  portions  of  old  roads, 
of  making  such  changes  in  position  when  improving  a 
road  as  may  tend  to  reduce  the  cost  of  conducting 
traffic  over  it  and  render  it  more  convenient  and 
pleasant  for  the  use  of  travel,  or  of  determining  the 
details  of  alignment  and  grade  upon  a  new  road  which 
is  approximately  fixed  in  position  by  the  purpose  of 
its  construction. 

The  most  economical  location  is  that  for  which  the 
sum  of  the  annual  costs  of  transportation,  the  annual 
costs  for  maintenance,  and  the  interest  on  the  cost  of 
construction  is  a  minimum. 

The  cost  of  conducting  transportation  is  affected  by 
the  rate  of  grade  of  the  road,  the  amount  of  rise  and 
fall  in  it,  and  the  length  of  the  road.  The  rate  of 
grade  is  important,  because  it  limits  the  loads  that  can 
be  hauled  over  the  road,  or  determines  the  number  of 
loads  that  must  be  made  to  transport  a  given  weight 
of  freight,  as  well  as  fixes  a  limit  to  the  speed  of  travel. 
The  amount  of  rise  and  fall  affects  the  expenditure  of 
power  required  to  haul  a  load  over  the  road.  The 
length  of  the  road  has  an  effect  upon  the  amount  of 
work  necessary  to  haul  a  load  over  it,  the  time  required 
for  a  trip,  and  the  cost  of  maintaining  the  road  surface; 
each  of  which,  other  conditions  being  the  same,  is 
directly  proportional  to  the  length. 

The  cost  of  construction  depends  upon  the  accuracy 
with  which  the  line  of  the  road  is  fitted  to  the  surface 


£2         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

of  the  ground,  as  determining  the  amount  of  earth- 
work and  cost  of  bridges  and  culverts;  upon  the 
character  of  the  ground  over  which  the  road  is  to  be 
built,  which  affects  the  cost  of  executing  the  work  and 
determines  the  necessity  for  and  expense  of  drainage; 
and  upon  the  cost  of  land  for  right  of  way.  All  of  these 
items  must  be  considered  in  any  comparison  of  the 
cost  of  constructing  on  various  routes.  Special  care 
should  be  taken  in  selecting  a  line  to  avoid  bad  ground, 
such  as  swamps,  upon  which  construction  may  be  diffi- 
cult and  expensive.  The  availability  near  the  line  of 
the  road  of  materials  needed  for  surfacing  may  also 
become  a  matter  of  importance  in  the  cost  of  construc- 
tion, and  have  an  influence  in  determining  location. 

The  relative  importance  of  the  various  elements 
affecting  the  choice  of  a  line  depends  upon  the  nature 
and  amount  of  the  traffic  to  be  provided  for  and  upon 
the  character  of  the  road  surface  to  be  used.  Where 
the  traffic  is  heavy,  the  importance  of  reducing  the 
cost  of  moving  it  by  lessening  grades  and  distance  will 
be  greater  than  where  the  traffic  is  light,  and  the  cost 
of  construction  may  be  correspondingly  increased  for 
that  purpose.  If  a  smooth  surface  be  employed,  upon 
which  traction  is  light,  the  value  of  reducing  grades 
will  be  greater  and  the  value  of  reducing  distance  less 
than  with  a  surface  of  poorer  tractive  qualities. 

ART.  17.     LENGTH  OF  ROAD. 

Changes  in  the  length  of  a  road  affect  all  portions  of 
the  traffic  in  the  same  manner,  and  the  expenditure  of 
power  and  loss  or  gain  in  time  occasioned  by  them  are 
in  general  directly  proportional  to  their  amounts. 

The  value  of  any  considerable  saving  in  length  may 
usually  be  considered  as  equal  to  the  same  percentage 


LOCATION   OF   COUNTRY   ROADS.  53 

of  the  whole  cost  of  conducting  the  traffic  that  the 
saving  in  distance  is  of  the  whole  length.  If,  therefore, 
a  rough  estimate  may  be  made  of  the  annual  traffic  to 
be  expected  upon  a  given  line  of  road  and  of  the  cost 
of  carrying  the  traffic,  this  cost  divided  by  the  length 
in  miles  through  which  the  traffic  is  moved  will  give 
the  annual  interest  upon  the  sum  that  may  reasonably 
be  expended  in  shortening  the  road  one  mile,  or  upon 
the  value  of  a  saving  of  a  mile  in  distance;  or  dividing 
by  the  number  of  feet  of  distance  will  give  the  value  of 
saving  one  foot. 

It  is  to  be  noted,  however,  that  the  cost  of  the  work 
of  transportation  is  not  necessarily  proportional  to  the 
amount  of  work  done,  and  consequently  this  method 
would  not  be  strictly  accurate  even  were  the  data  as 
to  traffic  and  costs  readily  obtainable.  An  estimate  of 
this  character  at  best  amounts  to  only  a  rough  guess, 
but  it  may  often  be  of  use  as  an  aid  to  the  judgment 
in  deciding  upon  the  value  of  a  proposed  improvement 
involving  a  considerable  change  of  length  in  a  road. 

Where  the  road  is  so  situated  and  the  saving  in 
distance  proposed  is  such  that  it  would  enable  teams 
to  make  an  additional  trip  per  day  in  the  hauling  of 
freight,  the  difference  in  cost  of  transportation  is  quite 
tangible  and  readily  estimated;  but  where  the  traffic  is 
of  a  more  indefinite  nature,  or  the  saving  proposed 
insufficient  to  admit  of  additional  trips,  the  value  of 
the  difference  of  length  depends  upon  the  value  to 
other  work  of  the  small  portions  of  time  of  men  and 
teams  which  may  be  saved  by  the  shorter  route  —  a 
value  which  exists,  but  is  difficult  to  estimate. 

There  is  also  a  value  in  the  saving  of  distance  due 
to  the  advantage  to  the  community  of  bringing  t!:e 
various  points  closer  together,  such  as  bringing  two 


54         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

towns  into  closer  relations  or  bringing  country  property 
nearer  to  markets.  The  method  of  considering  the 
cost  as  proportional  to  the  work  done  will  therefore 
probably  give  a  fair  idea  of  the  actual  economy  in  any 
saving  in  the  work  of  transportation. 

The  value  of  reducing  distance  varies  with  the 
character  of  the  road  surface.  As  the  cost  of  transpor- 
tation is  less  over  a  smooth  than  over  a  rough  surface, 
on  account  of  the  lighter  traction,  the  value  of  reduc- 
ing distance  is  also  less  on  the  smooth  surface. 

The  value  of  saving  distance  also  is  greater  on  a 
road  where  the  ruling  gradients  are  steep  than  upon 
one  with  light  gradients,  because  of  the  greater  num- 
ber of  loads  necessary  to  move  the  same  traffic. 

The  cost  of  maintenance  of  a  road  varies  \vith  its 
length,  and  under  similar  conditions  may  be  con- 
sidered, like  the  cost  of  transportation,  to  be  directly 
proportional  to  the  length  of  road. 

The  saving  in  cost  of  maintenance  from  decreasing 
distance  must  of  course  be  added  to  that  in  cost  of 
transportation  in  order  to  find  the  actual  value  of  a 
change  of  length. 

The  value  of  straightness  for  a  country  road  is  fre- 
quently very  much  overrated.  Considerable  devia- 
tions from  the  straight  line  may  often  be  made  with 
but  slight  increase  in  length,  and  there  seems  to  be  no 
good  reason  for  insisting  upon  absolute  straightness. 
The  error  is  commonly  made  of  sacrificing  grade  and 
expense  in  construction  to  the  idea  of  straightness 
without  the  attainment  of  any  considerable  saving  in 
length. 

It  involves  in  many  cases  the  injury  of  the  beauty  of 
the  road  and  of  the  landscape,  with  no  compensating 
economic  advantages. 


LOCATION  OF  COUNTRY  ROADS.        55 

ART.  1 8.     RISE  AND  FALL. 

'By  the  amount  of  rise  and  fall  is  meant  the  total 
vertical  height  through  which  a  load  must  be  lifted  in 
passing  in  each  direction  over  the  road.  It  is  distinct 
from  and  independent  of  the  rate  of  gradient. 

The  minimum  amount  of  rise  and  fall  is  found 
where  the  rise  is  all  in  one  direction  and  the  fall  in 
the  other,  each  being  equal  to  the  difference  of  eleva- 
tion of  the  terminal  points.  Any  increase  in  the  rise 
and  fall  beyond  this  amount  is  represented  by  the  rise 
encountered  in  passing  from  the  higher  to  the  lower 
terminus.  This  may  be  considered  as  avoidable  rise 
and  fall.  If  the  cost  of  developing  the  work  necessary 
to  overcome  rise  and  fall  be  the  same  as  that  of  develop- 
ing an  equal  amount  of  work  to  overcome  distance,  the 
rise  and  fall  may  be  evaluated  in  terms  of  distance, 
and  any  change  in  rise  and  fall  may  be  considered  as 
though  it  were  a  difference  in  distance  and  treated  as 
in  Art.  17. 

The  value  of  rise  and  fall  in  terms  of  distance  will 
depend  upon  the  nature  of  the  road  surface,  as  the 
work  necessaty  to  lift  a  given  load  to  a  given  height  is 
a  constant,  while  the  work  done  in  hauling  a  load  over 
a  given  distance  will  vary  with  the  resistance  offered  to 
traction  by  the  surface.  Thus,  taking  the  surface  as 
above,  the  work  of  lifting  one  ton  through  a  rise  of 
I  foot  is  2000  foot-pounds,  while  with  a  tractive  force 
of  100  pounds  per  ton  2000  +  I  oo  =20  feet,  the 
distance  a  ton  may  be  moved  on  the  level  surface  in 
developing  2000  foot-pounds  of  work.  Therefore  I 
foot  of  rise  or  fall  may  be  considered  as  equivalent  to 
20  feet  of  level  distance,  and  the  value  of  reducing  the 
amount,  of  rise  and  fall  may  be  found  from  that  for 


56        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

reducing  distance.  If  the  road  considered  were  a 
first-class  macadam  road,  with  resistance  of  40  pounds 
per  ton,  I  foot  of  rise  or  fall  would  equal  2000  -=-  40  = 
50  feet  of  distance. 

Where  the  rate  of  grade  is  less  than  the  angle  of 
repose  of  the  wheels  upon  the  road  surface  (see  Art.  2) 
no  additional  work  is  imposed,  by  avoidable  rise'  and 
fall,  upon  teams  hauling  loads  over  the  road.  The 
amount  of  work  done  in  lifting  the  loads  up  the  rise 
is  equal  to  that  done  by  the  grade  in  diminishing  trac- 
tion in  descending  the  fall,  and  the  total  work  required 
is  equal  to  that  necessary  to  haul  the  loads  from  one 
terminus  to  the  other  upon  a  uniform  gradient.  Upon 
an  undulating  road,  therefore,  where  the  grades  are 
light,  there  is  no  economic  advantage  in  reducing  the 
rise  and  fall  of  the  road. 

When  the  rate  of  grade  is  greater  than  the  angle  of 
repose,  the  amount  of  work  imposed  by  avoidable  rise 
and  fall  is  equal  to  twice  that  caused  by  the  excess 
of  fall  above  that  at  the  angle  of  repose.  In  this 
case  an  additional  amount  of  work  must  be  done  in 
applying  a  resistance  to  prevent  the  too  rapid  descent 
of  the  vehicle  in  going  down  the  grade.  The  amount 
of  this  work  in  any  case  equals  the  work  done  in  lifting 
the  load  to  a  height  equal  to  the  difference  between  the 
actual  rise  of  the  grade  in  question  and  the  rise  of  a 
grade  of  the  same  length  and  a  rate  equal  to  the  angle 
of  repose.  Thus  on  an  ordinary  earth  road  whose 
resistance  to  traction  where  level  is  1 00  pounds  per 
ton,  suppose  a  grade  to  occur  of  8  feet  per  1 00,  1 000 
feet  in  length.  For  the  road  surface  we  have  IOO-T- 
2000  =  .05,  and  the  angle  of  repose  is  a  5  per  cent  grade. 
Then  8  per  cent  —  5  per  cent  =  3  per  cent,  or  the 
brake-power  necessary  to  secure  uniform  motion  is 


LOCATION   OF   COUNTRY  ROADS.  57 

the  same  as  would  be  necessary  to  haul  the  load  up  a 
3  per  cent  grade,  and  a  grade  of  3  in  1 00  for  1000  feet 
gives  30  feet.  The  work  to  be  done  in  holding  back 
the  load  for  the  1000-ft.  grade  is  therefore  the  same  as 
would  lift  the  load  through  a  vertical  height  of  30  feet, 
or  the  fall  of  8  feet  per  100  for  1000  feet  has  the  same 
effect  as  30  feet  of  rise  in  the  same  direction,  pro- 
vided brake-power  costs  the  same  as  animal  power. 
The  work  saved  to  the  traffic  passing  down  this  grade, 
by  eliminating  it  as  avoidable  rise  and  fall  (without 
changing  the  ruling  gradients),  would  be  twice  the  above 
amounts  or  equal  to  lifting  the  loads  through  60  feet 
of  rise. 

ART.  19.     RATE  OF  GRADE. 

The  effect  of  any  change  in  the  ruling  gradient  upon 
a  road  depends  to  a  considerable  extent  upon  \vhat 
portion  of  the  traffic  may  be  carried  in  full  loads.  The 
lighter  portions  of  the  traffic  are  not  so  seriously 
affected  by  heavy  gradients  as  the  heavy  portions, 
although  there  is  an  advantage  in  light  gradients  for 
any  driving.  The  rate  of  speed  which  may  be  employed 
will  be  less  upon  the  portions  of  the  road  having  heavy 
grade,  and  the  time  occupied  in  a  trip  over  the  road  is 
therefore  affected  somewhat  by  the  rate  of  grade. 

The  desirability  of  a  road  for  general  driving  is  also 
much  influenced  by  the  gradients  employed,  as  is 
that  value  of  the  road  which  has  for  a  basis  the  effect 
it  may  exert  upon  the  attractiveness  of  the  locality. 
These  things  all  have  a  certain  financial  value,  which 
of  course  it  is  quite  impossible  to  estimate  with  any 
degree  of  accuracy,  but  which  should  be  considered  in 
determining  the  allowable  maximum  gradient  in  any 
case  in  practice. 


58         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

For  heavy  traffic,  such  as  the  transfer  of  goods  from 
one  town  to  another  or  the  marketing  of  country  prod- 
uce, the  limitation  of  load  placed  upon  the  traffic  by 
the  gradient  is  a  matter  of  importance,  the  effect  of 
which  is  calculable  upon  the  cost  of  transportation. 
If  in  any  case  the  approximate  amount  of  this  heavy 
traffic  which  is  likely  to  be  carried  in  full  loads  be  deter- 
mined, the  relative  costs  of  its  transportation  over 
two  lines  of  differing  gradient,  other  conditions  being 
similar,  will  be  nearly  proportional  to  the  number  of 
loads  required  to  move  the  traffic  over  each  gradient. 

In  estimating  the  value  of  reducing  the  rate  of  grade, 
it  may  be  considered,  as  in  the  case  of  a  reduction  of 
length,  that  its  value  to  the  community  is  represented 
by  the  saving  in  annual  costs  of  transportation,  and 
that  the  amount  that  may  reasonably  be  expended  in 
increased  cost  of  construction  to  effect  a  reduction  of 
gradient  is  the  sum  upon  which  this  annual  saving  is 
the  interest. 

The  length  of  a  road  and  the  amount  of  rise  and 
fall  on  it  determine  the  amount  of  work  that  must  be 
done  in  hauling  a  load  over  the  road.  The  rate  of 
gradient,  on  the  contrary,  does  not  affect  the  amount 
of  work  necessary  to  move  the  traffic,  but  it  limits  the 
load  that  a  horse  may  haul  at  one  trip. 

The  establishment  of  a  proper  rate  for  the  ruling 
grade  of  the  line  is,  therefore,  usually  the  most  impor- 
tant point  in  location.  In  localities  where  light  gra- 
dients are  easily  obtained  the  problem  of  location  is 
greatly  simplified. 

By  referring  to  Art.  3  the  comparative  loads  that 
a  horse  may  draw  up  different  grades  will  give  some 
idea  of  the  importance  of  carefully  considering  the 
question  of  gradient.  In  nearly  all  cases  in  practice 


LOCATION  OF  COUNTRY  ROADS.         59 

there  is  a  considerable  latitude  within  which  gradients 
may  be  chosen.  It  is  usually  a  question  of  heavier 
gradients  as  against  greater  distance  and  larger  first 
cost  for  the  road.  It  may  be  remarked  that  it  is  only 
under  exceptional  circumstances  that  it  is  either  neces- 
sary or  advisable  to  use  a  steeper  gradient  than  5  per 
cent  on  the  new  location  of  a  country  road  of  any 
importance.  Grades  steeper  than  the  ruling  gradient 
may  sometimes  be  introduced  over  short  distances 
without  impairing  the  efficiency  of  the  road,  as  horses 
are  usually  able  to  exert  for  a  short  time  a  force  much 
greater  than  they  can  continuously  exert.  If  the 
length  of  grade  be  quite  short,  200  or  300  feet,  a  horse 
can  about  double  his  ordinary  power  in  passing  it. 

Where  long  steep  grades  must  be  used,  it  is  desirable 
to  break  them  by  short  stretches  of  lighter  gradients  to 
provide  resting-places  for  horses. 

Heavy  gradients  also  have  the  disadvantage  of 
retarding  traffic  in  the  direction  of  falling  grade,  and, 
as  suggested  in  Art.  1 8,  of  requiring  the  expenditure 
of  work  to  hold  the  load  from  too  rapid  descent. 

ART.  20.     EXAMINATION  OF  COUNTRY. 

For  the  purpose  of  obtaining  the  requisite  data 
upon  which  to  base  the  location  of  a  road,  it  is  neces- 
sary that  a  careful  examination  be  made  of  the  topo- 
graphical features  of  the  country  through  which  the 
line  is  to  pass.  The  relative  elevations  of  the  termini 
of  the  line  and  of  intermediate  points  should  be 
obtained,  and  the  directions  and  steepnesses  of  the 
various  natural  slopes  determined. 

If  a  line  were  to  be  located  connecting  points  at  long 
distances  from  each  other,  as  sometimes  occurs  in 


60         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

railway  location,  it  would  be  necessary  to  study  the 
general  configuration  of  the  country,  noticing  the 
direction  of  flow  of  the  streams,  and  the  location  and 
elevations  of  the  various  passes  in  the  ridges  through 
which  it  might  be  possible  to  carry  the  line.  Usually 
it  would  be  found  that  the  country  is  composed  of  a 
series  of  valleys,  separated  by  ridges,  branching  in  a 
systematic  manner  from  the  main  watercourse  of 
the  region,  and  that  the  passes  in  the  ridges  occur  at 
the  head  of  side  streams,  and  especially  where  streams 
flowing  into  valleys  on  opposite  sides  of  the  ridge  have 
their  sources  near  each  other. 

In  the  location  of  common  roads,  however,  the  prob- 
.lem  is  ordinarily  of  a  less  extended  nature,  and  may 
consist  in  joining  two  points  lying  in  the  same  valley, 
or  in  joining  points  in  adjacent  valleys  by  a  line  pass- 
ing over  a  ridge.  In  these  cases  it  is  only  necessary  to 
take  into  account  the  slope  of  the  valleys  in  question, 
the  positions  and  elevations  of  available  passes,  and  the 
side  slope  of  the  ridges. 

The  slope  of  the  bed  of  a  valley,  in  hilly  country, 
usually  forms  a  concave  curve,  the  rate  of  slope  gradu- 
ally increasing  from  the  lower  to  the  upper  end.  In  a 
valley  of  considerable  length  this  increase  in  the  rate 
of  slope  may  be  very  gradual  or  in  short  valleys  rising 
to  a  considerable  height  it  may  be  more  sudden.  The 
profile  ABCD  in  Fig.  n  shows  the  slope  of  a  short 
valley  which  decreases  in  slope  from  about  ten  feet 
per  hundred  at  the  upper  end  to  about  two  feet  per 
hundred  at  the  lower  end. 

When  a  map  of  the  country  to  be  traversed  is  avail- 
able, showing  the  positions  and  elevations  of  the  points 
controlling  the  location,  the  work  is  very  much  simpli- 
fied, the  reconnaissance  may  for  the  most  part  be 


LOCATION   OF   COUNTRY  ROADS. 


61 


62         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

limited  to  a  study  of  the  map,  and  the  routes  may  be 
sketched  upon  the  map  to  be  tried  in  the  field.  If  the 
map  at  hand  is  an  accurate  contour  map  on  a  sufficiently 
large  scale,  the  entire  location  may  be  worked  out  in 
detail  upon  the  map,  leaving  only  the  work  of  staking 
out  the  line  to  be  done  upon  the  ground. 

Maps  may  be  obtained,  in  most  parts  of  this  country, 
upon  which  the  horizontal  positions  of  points  may  be 
readily  fixed  with  sufficient  accuracy  for  the  purposes 
of  the  preliminary  examination.  Where  such  maps  are 
not  obtainable,  the  positions  of  points  must  be  ascer- 
tained and  a  rough  map  prepared.  For  this  purpose 
directions  may  be  measured  with  a  pocket  compass, 
and  distances  estimated  or  obtained  by  the  use  of  an 
odometer  or  pedometer,  as  may  be  most  convenient. 

Differences  of  elevation  are  easily  obtained  with  a 
fair  degree  of  accuracy  by  the  use  of  an  aneroid 
barometer,  and  slopes  may  be  measured  with  a  hand 
level. 

Where  the  rough  means  ordinarily  employed  in  the 
reconnaissance  are  not  sufficiently  accurate  to  deter- 
mine the  controlling  points  of  the  lines  to  be  adopted, 
a  more  complete  examination  of  the  country  may  often 
be  made  by  a  rapid  topographical  survey  by  means  of 
the  transit  and  stadia  method. 

Whatever  means  may  be  adopted  for  doing  the 
work,  the  preliminary  examination  should  determine  a 
map  showing  the  approximate  positions  of  the  con- 
trolling points  through  which  the  road  must  pass,  and 
enable  a  rough  sketch  to  be  made  of  the  slopes  of  the 
country  through  which  the  line  is  to  be  run. 


LOCATION    OF    COUNTRY    ROADS.  63 

ART.  21.     PLACING  THE  LINE. 

After  the  preliminary  examination  of  the  locality  i$ 
complete  and  the  positions  and  elevations  of  the  con- 
trolling points  of  the  line  are  known  with  reference  to 
each  other,  the  line  must  be  selected  and  run  in  upon 
the  ground,  or,  if  the  reconnaissance  is  not  conclusive 
as  to  the  position  of  the  best  line,  it  is  advisable  to 
run  in  two  or  more  lines  and  make  a  more  detailed 
comparison  between  them. 

The  controlling  points  of  a  line  are  those  points  at 
which  the  position  of  the  road  is  restricted  within 
narrow  limits,  and  is  not  subject  to  change.  These 
ma}T  be  points  where  the  location  is  governed  by  the 
necessity  of  providing  for  traffic,  or  points  where  the 
position  of  the  line  is  restricted  by  topographical  con- 
siderations, such  as  a  summit  over  which  the  line  is  to 
pass  a  ridge  or  a  favorable  location  for  a  bridge. 

Where  the  line  is  to  be  located  to  a  uniform  gradi- 
ent, it  should  be  started  from  the  controlling  point  at 
the  end  of  the  grade,  which  is  usually  the  summit.  It 
is  then  laid  off  along  the  slope  in  such  manner  as  to 
cause  it  to  have  continuously  the  rate  of  grade  decided 
upon.  Taking  D  (Fig.  n)  at  the  summit  of  the  valley 
as  the  controlling  point,  it  is  seen  that  the  distance 
from  C  to  D  is  sufficient  to  give  a  gradient  of  10  in 
100  by  following  directly  down  the  valley,  and  the  line 
with  that  gradient  may  be  run  in  that  manner. 

The  maximum  gradient  from  A  to  C  is,  however, 
only  5  in  100,  and  if  thought  advisable  the  same  maxi- 
mum gradient  may  be  used  between  C  and  D  by  run- 
ning the  line  DHC  diagonal!}'  down  the  slope,  as 
shown.  This  line,  having  one-half  the  gradient,  will 
have  about  twice  the  length  of  the  line  CD. 


64          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

In  running  this  line  it  is  started  from  the  highest 
point  of  maximum  grade,  and  points  at  the  surface  of 
the  ground  are  continually  selected,  in  advance  of  the 
placing  of  the  line,  which  are  at  the  proper  elevation 
to  permit  the  grade  to  pass  through  them.  This  may 
be  accomplished  by  setting  off  the  angle  of  the  gradi- 
ent upon  the  vertical  circle  of  the  transit,  or  upon  a 
gradienter,  and  sighting  upon  a  rod  which  is  moved 
until  the  line  of  sight  strikes  it  at  the  same  height  from 
the  ground  that  the  instrument  is  setting.  The  points 
for  the  line  may  also  be  found  by  running  a  line  of 
levels  ahead  of  the  transit  line  (a  hand  level  is  conven- 
ient for  this  purpose)  and  pacing  distances  upon  which 
to  reckon  the  gradient,  the  distances  and  elevations 
being  frequently  checked  upon  those  of  the  measured 
line. 

The  location  of  a  gradient  upon  a  common  road 
differs  from  that  upon  a  railroad  only  in  that  steeper 
gradients  are  used,  sharper  curves  or  angles  may  be 
employed,  and  the  gradients  need  not  be  lessened  on 
ordinary  bends  or  curves.  If  the  line  is  to  make  a 
turn  upon  the  slope  as  at  H,  the  grade  should  be 
flattened  at  the  turn,  and  a  curve  of  as  large  radius  as 
possible,  without  too  great  expense  for  grading,  be 
introduced. 

In  a  manner  similar  to  the  above  a  line  might  be 
run  from  D  on  the  other  side  of  the  valley,  which 
using  a  5  per  cent  gradient  would  give  the  line  DML, 
reaching  the  bed  of  the  valley  at  the  point  L.  A  lighter 
gradient  may  be  obtained  from  A  to  D  by  starting 
from  D  and  going  down  by  a  continuous  gradient  of 
4  in  100  on  the  line  DFGA,  and  greater  or  less  rates 
of  descent  may  be  adopted  and  lines  corresponding  to 
them  located,  as  may  be  considered  advisable. 


LOCATION   OF   COUNTRY  ROADS.  65 

The  center-line  for  a  final  location  should  be  care- 
fully run,  and  points  permanently  marked  from  which 
it  may  be  relocated  when  necessary.  An  accurate  line 
of  levels  should  also  be  run  over  the  center-line  and  a 
profile  drawn,  upon  which  the  grades  may  be  estab- 
lished and  earthwork  estimated. 

After  placing  the  center-line,  topograplw  should  be 
taken  carefully  upon  each  side  of  the  line  for  some 
distance,  and  a  map  drawn  showing  the  topography 
and  giving  elevations  by  means  of  contours.  This  will 
serve  to  show  whether  the  line  is  placed  to  the  best 
advantage,  and  whether  any  changes  are  desirable. 
This  is  especially  necessary  over  rough  ground  or 
where  the  line  is  on  maximum  gradient,  as  frequently, 
and  perhaps  usually,  the  first  line  run  will  be  useful 
only  as  a  preliminary  line,  which  with  its  accompany- 
ing topography  will  permit  a  proper  location  to  be 
made. 

ART.  22.     COMPARISON*  OF  ROUTES. 

In  selecting  a  line  for  the  construction  of  a  road  the 
principles  already  mentioned  in  the  early  part  of  this 
chapter  should  be  had  in  mind.  The  line  must  be  well 
designed  to  accommodate  the  traffic.  It  should  have 
as  easy  grades,  short  length,  and  small  rise  and  fall  as 
is  consistent  with  a  reasonable  cost  of  construction,  in 
order  to  give  light  costs  for  transportation  and  for 
maintenance. 

Suppose  in  the  case  shown  in  Fig.  1 1  that  it  is  desired 
to  connect  the  village  at  the  point  A  with  the  point  D 
and  with  the  roads  leading  through  the  passes  at  F 
and  /.  Which  line  it  will  be  the  most  advantageous 
to  adopt  depends  upon  the  relative  importance  of  the 
traffic  to  the  various  points  considered. 


66         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

The  shortest,  and  probably  cheapest,  line  from  A 
to  D  would  be  obtained  by  following  the  valley  over 
the  line  A  BCD,  which  line,  as  shown  by  the  profile, 
would  give  a  maximum  gradient  of  10  in  1 00  between 
C  and  D.  The  line  FB  joining  the  first  line  at  B 
would  afford  communication  with  the  summit  at  F 
with  a  maximum  gradient  of  5  in  100.  If  the  traffic  to 
the  point  D  be  small  and  unimportant,  so  that  addi- 
tional expense  in  reducing  the  gradient  from  C  to  D 
is  unadvisable,  these  lines  might  prove  a  satisfactory 
location. 

If,  however,  D  be  a  point  of  importance  and  the 
traffic  from  A  to  D  heavy,  it  will  be  necessary  to  adopt 
some  means  to  reduce  the  gradient  from  C  to  D. 
Leaving  out  of  consideration  the  point  F  and  consider- 
ing B  and  C  as  points  of  minor  importance,  it  might 
be  advisable  to  use  the  line  ALMD  with  a  uniform 
5  per  cent  gradient  from  D  to  L,  and  branches  to 
connect  with  C  and  B.  This  would  give  a  line  but 
little  longer  than  the  valley  line,  with  only  one-half  the 
ruling  gradient  of  that  line. 

If  C  is  not  important  and  can  be  neglected  while  B 
and  F  must  be  considered,  the  line  ABEHD  has  a 
maximum  gradient  of  5  in  1 00,  and  connects  A  with 
the  points  BF  and  D  with  a  minimum  total  length  of 
road  (being  less  than  the  valley  line  first  considered). 

When  B  and  C  must  both  be  considered  as  of  im- 
portance as  well  as  F  and  D,  the  lines  ABCHE  and 
HD  will  give  a  ruling  gradient  of  5  in  100  to  both  F 
and  D,  and  passing  through  B  and  C  with  a  somewhat 
longer  line  than  in  the  last  case. 

This  arrangement  would  make  the  length  of  haul 
from  A  to  D  and  F  each  longer  than  by  the  first  line 
considered;  but  the  gradient  to  D  would  be  lighter,  and 


LOCATION   OF   COUNTRY  ROADS.  67 

the  total  length  of  road  to  be  constructed  and  main- 
tained would  be  less. 

In  case  the  points  B  and  C  are  both  unimportant, 
and  the  line  through  the  valley  may  be  neglected,  the 
line  AGFD  provides  a  ruling  gradient  of  4  in  100  from 
A  to  both  F  and  D,  and  connects  them  with  each 
other,  with  about  the  same  length  as  the  shortest  5  per 
cent  gradient.  When  the  point  /  must  be  taken  into 
account,  this  line  may  be  connected  with  /  by  the  line 
GI  having  a  gradient  of  4  in  1 00.  This  would  give  the 
shortest  line  of  uniform  gradient  to  connect  A  with  the 
three  points  I,  F  and  D,  and  possibly  a  desirable  line  to 
construct  when  the  line  through  the  point  /  is  impor- 
tant, even  if  the  valley  road  from  A  to  B  is  also  neces- 
sary. 

The  lines  upon  the  side  slopes  are  usually  more  ex- 
pensive to  construct  than  the  valley  lines,  and  the  dif- 
ferences of  first  cost  of  the  various  lines  must  of  course 
be  considered.  The  importance  of  a  difference  in  ex- 
pense of  construction  depends  upon  the  traffic  to  be 
hauled  over  the  road  and  the  kind  of  surface  to 
be  used.  Where  a  broken-stone  or  gravel  road  is  to  be 
constructed  at  considerable  expense,  the  difference  of 
cost  due  to  a  change  of  location  is  relatively  less 
important  as  being  a  less  percentage  of  the  whole  cost, 
while  the  difference  of  tractive  effort  due  to  grade  is 
of  more  importance,  as  being  a  higher  percentage  of 
that  upon  the  level,  than  would  be  the  case  with  an 
ordinary  earth  road. 

As  is  easily  seen  from  the  above  the  choice  of  a 
location  for  a  road,  while  depending  upon  principles 
easily  stated,  is  in  reality  a  matter  requiring  the  use  of 
judgment,  and  is  not  readily  reducible  to  a  financial 
comparison  stated  in  money  values,  because  the  data 


68         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

concerning  the  volume  of  the  traffic  and  the  cost  of 
conducting  it  can  be  determined  only  very  roughly, 
and  contains  many  elements  of  error.  For  purposes  of 
comparison  to  aid  the  judgment,  approximate  data 
may  often  be  assumed  or  determined  by  a  study  of  the 
localities  affected.  In  some  cases  observations  may  be 
made  of  the  number  of  teams  of  different  classes  pass- 
ing certain  points  within  certain  times,  to  give  a  basis 
for  estimation  of  the  annual  volume  of  traffic.  In 
other  cases,  the  annual  hauling  traffic,  which  is  usually 
the  most  important  portion  of  the  traffic  in  considering 
location,  may  be  estimated  from  the  known  interests  of 
the  locality.  Thus,  if  the  produce  of  a  certain  section 
of  farming  country  must  be  hauled  over  a  given  road 
to  market,  the  amount  of  this  produce  may  be  esti- 
mated from  the  acreage,  and  the  relative  number  of 
loads  upon  different  grades  then  determined.  The 
cost  per  load  over  the  road  would  then  need  to  be  as- 
sumed in  order  to  find  the  annual  value  of  a  reduction 
of  grade. 

In  the  same  manner,  the  effect  of  changes  of  length 
and  in  the  amount  of  rise  and  fall  may  be  found  as 
indicated  in  Arts.  17  and  18. 

All  of  these  items  must  be  combined  to  find  the  rela- 
tive total  costs  of  transportation  for  each  route.  The 
cost  of  construction  and  of  maintenance  for  each  line 
must  then  be  estimated,  and  that  line  is  the  most  ad- 
vantageous which  makes  the  sum  of  the  annual  charges 
and  the  interest  on  the  first  cost  a  minimum.  Where 
several  lines  of  traffic  are  to  be  considered  together  as 
in  Fig.  ii,  the  cost  of  conducting  all  of  the  traffic  by 
each  system  of  lines  that  may  be  employed  must  be 
considered,  the  entire  cost  being  made  a  minimum  for 
the  system  to  be  adopted. 


LOCATION  OF  COUNTRY  ROADS.  69 

ART.  23.     CHANGING  EXISTING  LOCATIONS. 

The  problem  that  arises  oftener  than  any  other 
in  country-road  location  is  that  of  improving  short 
stretches  of  road,  where,  owing  to  defective  location, 
the  grades  are  unnecessarily  heavy,  the  length  unneces- 
sarily great,  or  the  ground  over  which  the  road  may 
pass  such  as  to  make  its  maintenance  in  good  con- 
dition difficult  and  expensive.  The  first  of  these  is 
the  most  common  defect  of  ordinary  country  roads,  as 
shortness  of  distance  has  very  commonly  been  obtained 
by  the  disregard  of  the  desirability  of  light  gradients, 
which  in  very  many  cases  are  easily  obtainable. 

The  principles  to  be  observed  and  .methods  of  pro- 
cedure in  making  the  new  location  are  exactly  the 
same  as  in  an  original  location,  save  that  in  this  case  a 
road  already  exists,  and  the  question  of  economy  is  one 
of  determining  whether  the  advantages  to  be  gained 
in  lessened  costs  of  transportation  and  maintenance 
is  sufficient  to  warrant  the  expense  of  obtaining  new 
right  of  way  and  constructing  new  road. 

In  Fig.  12  is  given  an  example  that  is  frequently  met 
in  practice,  where  the  existing  road  abed  runs  over  the 
point  of  a  hill,  with  heavy  gradient,  while  a  line  of  very 
much  lighter  gradient  might  be  located  around  the 
base  of  the  hill  through  the  pass  at  e,  giving  a  greater 
length  of  road,  but  much  less  rise  and  fall.  The  line 
bed  in  the  figure  has  a  length  about  800  feet  greater,  a 
rise  and  fall  70  feet  less,  and  a  maximum  gradient  one 
half  as  steep  as  the  line  bed.  These  relations  are  shown 
in  the  profile  in  Fig.  12. 

If  the  road  in  question  be  a  common  earth  road, 
I  foot  of  rise  and  fall  may  be  taken  as  equivalent,  in  the 
work  required  to  haul  a  load  over  it,  to  20  feet  of  dis- 


70         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 


LOCATION   OF  COUNTRY  ROADS.  7 1 

tance,  and  the  70  feet  saved  by  the  new  location  would 
be  equivalent  to  1400  feet  of  distance.  Hence,  the 
line  bed  may  be  considered  as  having  an  equivalent 
length  for  purposes  of  traffic  1400  —  800  =  600  feet 
shorter  than  the  line  bed.  In  addition  to  this,  loads 
may  be  taken  over  the  new  line  in  direction  b  to  d 
more  than  double,  and  in  direction  from  d  to  b  triple, 
in  weight  those  that  can  be  taken  by  the  same  power 
over  the  old  line. 

A  further  improvement  of  the  line  may  also  be 
possible,  if  the  new  line  can  join  the  old  one  at  a  point 
lower  down  than  6,  by  running  a  lighter  gradient  than 
5  in  100  from  the  point  e.  Thus  the  line  efa  would 
give  an  uniform  gradient  of  4  per  cent,  but  would 
require  the  construction  of  more  new  line. 

In  considering  changes  of  location,  it  is  also  neces- 
sary to  take  into  account  the  interests  of  adjoining 
owners.  Houses  and  buildings  are  largely  located  with 
reference  to  the  existing  position  of  the  roads,  and 
changes  in  the  position  of  a  road  may  involve  injury  to 
such  property.  The  question  then  becomes  largely 
one  of  sacrificing  the  interests  of  the  users  of  the  road, 
or  those  of  the  adjoining  owners  —  a  question  that 
should  be,  but  commonly  is  not,  decided  by  consider- 
ing what  will  be  of  most  advantage  to  the  general  com- 
munity. 


CHAPTER  IV. 

IMPROVEMENT  AND  MAINTENANCE  OF  COUNTRY  ROADS. 
ART.  24.     NATURE  OF  IMPROVEMENTS. 

ORDINARY  country  roads  may  be  classified  as  earth 
roads,  gravel  roads,  and  broken-stone  roads.  The 
larger  number  of  common  roads  throughout  this  coun- 
try belong  of  necessity  to  the  first  class.  In  a  few  of 
the  more  enterprising  communities  the  more  important 
roads  are  constructed  of  gravel  or  broken  stone. 

The  percentage  of  roads  of  the  better  class  is,  how- 
ever, very  small,  and  although  there  has  recently  been 
a  distinct  improvement  in  this  particular,  the  inability 
of  rural  communities  to  at  once  raise  the  funds  neces- 
sary for  the  general  construction  of  first-class  new 
roads  will  cause  their  increase  to  be  very  gradual. 

Improvement  in  country  roads  may  be  of  several 
kinds: 

(1)  Changes  in  location,  by  which  better  alignment 
or  better  gradients  may  be  obtained,  or  by  which  the 
natural  conditions  of  surface  or  drainage  may  be  im- 
proved.    This  has  been  discussed  in  Chapter  III. 

(2)  Reconstruction  of  the  road-bed,  as  in  regrading 
steep  slopes  to  give  lighter  gradients,  or  in  raising  the 
road-bed   across   low   and   wet   places  to   provide   for 
drainage. 

(3)  The  construction  of  artificial  drainage  where  a 
road  is  built  over  ground  which  is  likely  to  become  soft 
in  wet  weather,  or  where  water  may  reach  the  road-bed 

72 


IMPROVEMENT  OF  COUNTRY  ROADS.      73 

from  underground  sources.  This  has  been  discussed  in 
Chapter  II. 

(4)  Improvement  of  the  surface,  which  may  consist 
in  re-forming  the  surface  of  natural  earth,  or  in  the 
construction  of  an  artificial  surface  or  pavement,  the 
latter  of  which  will  be  discussed  in  separate  chapters. 

The  more  important  lines  of  travel  leading  out  from 
the  towns  will  gradually  be  improved  by  the  con- 
struction of  broken-stone  or  other  permanent  roads,  but 
this  constitutes  but  a  small  percentage  of  the  total 
mileage,  and  the  problem  in  common-road  improve- 
ment is  for  the  most  part  that  of  making  the  most  of 
the  roads  that  exist,  rather  than  reconstructing  them 
with  new  material.  The  materials  and  funds  imme- 
diately available  must  be  used  to  secure  as  much  im- 
provement as  possible. 

Earth  roads,  under  the  most  favorable  conditions, 
do  not  usually  attain  a  high  degree  of  efficiency,  and 
are  not  economical  under  any  considerable  traffic. 
They  are,  however,  capable  of  much  improvement  and 
need  not  become,  as  they  frequently  do,  practically 
useless  during  a  large  portion  of  the  year.  This  im- 
provement must  be  gradual  and  come  about  through 
the  adoption  of  more  rational  methods  of  maintenance, 
rather  than  through  immediate  reconstruction  of  the 
road  surfaces. 


ART.  25.     GRADE  AND  CROSS  SECTION. 

As  already  explained  in  Art.  10,  the  drainage  of  the 
surface  of  a  road  is  accomplished  by  crowning  the  sur- 
face and  giving  it  a  proper  longitudinal  slope.  Under- 
drains  will  not  drain  water  from  the  surface  of  the  road, 
and  unless  the  crown  is  at  all  times  maintained  and  the 


74          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

surface  kept  smooth,  water  is  likely  to  stand  upon  the 
surface  and  soften  it. 

Grade.  The  surface  of  a  country  road  should  not  be 
level  in  the  direction  of  its  length,  but  should  have  a 
sufficient  longitudinal  slope  to  drain  any  water  from 
its  surface  which  might  otherwise  be  held  by  small  ruts 
or  depressions.  The  minimum  grade  for  an  earth 
road  should  be  at  least  \  foot  in  1 00  feet,  and  for  a 
broken-stone  road  nearly  as  much.  The  grade,  except 
in  very  rough  country,  should  not  exceed  4  or  5  feet 
per  100,  and  when  steeper  grades  are  necessary,  they 
should  be  made  as  light  as  may  be  feasible.  The  effect 
of  changing  the  rate  of  grade  is  discussed  in  Art.  19. 

Width.  Too  great  width  of  roadway  upon  country 
roads  causes  an  unnecessary  expense  in  the  con- 
struction and  maintenance  of  the  road,  and  the  width 
should  be  only  sufficient  to  provide  space  for  the 
easy  conduct  of  the  traffic.  For  roads  of  ordinary 
traffic,  this  requires  only  that  there  be  room  for  teams 
moving  in  opposite  directions  to  freely  pass.  A 
width  of  20  feet  is  ample  for  most  country  roads,  and 
for  roads  of  lighter  traffic  1 6  feet  is  often  sufficient. 
Outside  of  this  width,  side  ditches  must  be  formed  for 
carrying  the  surface  drainage. 

In  the  construction  of  gravel  or  broken-stone  roads, 


FIG.  13. 

the  paved  portion  of  the  road  does  not  usually  extend 
to  the  full  width  of  the  roadway,  a  shoulder  of  earth 
being  left  on  each  side,  as  shown  in  Fig.  13.  The  width 
of  broken  stone  on  ordinary  country  roads  may  vary 


IMPROVEMENT   OF   COUNTRY  ROADS.  75 

from  about  12  feet  to  1 6  feet.  A  greater  width  than 
this  need  only  be  employed  on  important  roads  which 
convey  large  traffic,  or  on  city  streets. 

While  the  improved  portion  of  the  road  should  be 
as  small  as  is  consistent  with  the  proper  discharge  of 
the  duty  required  of  it,  the  available  right  of  way  need 
not  be  so  restricted,  but  should  be  laid  out  wide 
enough  to  permit  of  the  widening  of  the  used  portion 
when  necessary,  and  allow  room  at  the  sides  for  pedes- 
trians, with  a  grass  border  and  line  of  trees.  When 
trees  are  planted  along  the  roadway  they  should  not 
be  placed  so  as  to  form  a  dense  shade  over  any  portion 
of  the  traveled  road,  although  a  moderate  shade  is 
not  a  disadvantage,  and  care  should  be  used  that  they 
are  not  near  enough  to  a  covered  drain  to  permit  the 
roots  to  grow  into  the  drain  and  choke  it. 

Crown.  The  surface  of  a  road  must  be  crowned 
sufficiently  to  cause  the  water  which  falls  upon  it  to 
run  at  once  into  the  gutters.  The  height  of  crown 
required  depends  upon  the  character  of  the  surface  and 
upon  the  grade  of  the  road.  A  high  crown  is  objec- 
tionable because  it  concentrates  the  travel  in  the  middle 
of  the  road,  which  tends  to  wear  hollows  longitudinally 
along  the  road  into  which  water  may  settle;  but  if 
the  crown  be  too  low,  small  depressions  worn  into  the 
surface  by  the  traffic  may  hold  water  and  cause  the 
road  to  become  soft.  The  slope  from  the  center  to 
the  side  of  an  earth  road  should  not  be  less  than  one 
in  twenty  nor  greater  than  one  in  ten,  corresponding 
to  a  height  of  crown  from  one-fortieth  to  one-twentieth 
of  the  width  of  the  road.  For  roads  upon  which  the 
surface  can  be  kept  in  smooth  condition  and  on 
moderate  grades,  the  lower  limit  may  be  used,  but  on 
the  average  country  road  a  steeper  slope  is  desirable 


?6         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

and  a  crown  of  one-twentieth  the  width  is  not  too 
great  to  secure  efficient  drainage. 

Gravel  and  macadam  roads  should  be  crowned 
somewhat  less  than  earth  roads.  On  wide  macadam 
streets  kept  in  smooth  condition,  the  side  slopes  may 
be  as  low  as  one  in  thirty,  or  a  crown  equal  to  one- 
sixtieth  of  the  street  width.  On  such  work,  however, 
it  is  more  common  and  probably  better  to  use  a  crown 
of  one-fiftieth,  or  even  one-fortieth  the  width;  while  on 
ordinary  graveled  or  macadamized  country  roads  the 
crown  should  be  from  one-thirtieth  to  one-twenty- 
fourth  of  the  width.  When  constant  attention  and 
careful  maintenance  can  be  relied  upon  to  keep  the 
surface  in  smooth  condition  a  less  crown  may  be  used 
than  would  be  allowable  if  the  road  is  likely  to  be 
subjected  to  considerable  wear  between  periods  when 
repairs  are  made. 

Form  of  Section.  There  is  considerable  difference  of 
opinion  amongst  road  builders  as  to  the  best  form  to 
give  the  surface  of  a  road.  Some  use  a  section  com- 
posed of  two  planes  of  equal  inclination  rounded  off  in 
the  middle  and  sloping  uniformly  to  the  sides  as  shown 
in  Fig.  14.  Others  prefer  to  use  a  convex  curve, 


FIG.  14. 

approximately  the  arc  of  a  circle,  or  more  commonly, 
a  parabolic  curve,  which  is  practically  identical  with 
the  circular  arc.  The  exact  form  is  not  a  matter  of 
importance  on  a  country  road,  and  either  of  them,  or 
some  intermediate  form,  may  give  good  results  in 
practice.  It  is  not  desirable  to  insist  upon  great 


IMPROVEMENT   OF  COUNTRY  ROADS.  ?/ 

accuracy  in  the  form  of  section  provided  a  proper 
crown  be  given  and  the  surface  be  properly  smoothed. 
Where  a  smooth  pavement  is  used,  however,  it  is 
desirable  to  place  it  accurately  to  a  uniform  section 
Gutters.  At  the  side  of  the  road  longitudinal  ditches 
must  be  provided  for  the  purpose  of  carrying  the  water 
drained  from  the  surface  of  the  road  to  some  point 
where  it  may  be  turned  into  a  natural  drainage  channel. 
In  many  instances  these  side  ditches  also  carry  the 
drainage  from  land  adjacent  to  the  road.  The  size 
and  form  of  the  gutters  will  naturally  depend  upon  the 
quantity  of  water  to  be  carried  and  the  slope  of  the 
gutters.  In  some  instances  the  extension  of  the  road 
surfaces,  as  shown  in  Fig.  13,  will  be  sufficient  and  no 
special  gutters  will  be  required.  In  deep  cuts  where 
the  excavation  necessary  to  form  side  ditches  would 
be  expensive,  a  tile  may  be  placed  under  each  side  of 
the  road,  as  shown  in  Fig.  1 6,  into  which  the  drainage 


FIG.  1 6. 

from  above  the  cut  and  from  the  small  gutters  may 
be  carried. 

Broad,  shallow  gutters  are,  in  general,  to  be  pre- 
ferred to  deep  and  narrow  ones.  The  side  slopes 
should  not,  in  any  case,  be  less  than  2  horizontal  to 
I  vertical,  and  4  or  5  to  I,  on  the  side  next  to  the  road- 
way, is  better.  Shallow  gutters  are  easier  to  form  and 
keep  clean,  and  are  not  so  likely  to  wash  out  at  times 
of  heavy  rainfall.  It  is  not  desirable  to  use  deep  side 


78         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ditches  for  the  purpose  of  under  drainage,  and  water 
will  not  be  drawn  from  the  surface  of  a  hollow  roadway 
into  such  ditches.  Fig.  17  shows  a  common  form 
where  it  is  intended  to  use  the  side  ditches  to  prevent 
any  seepage  of  water  from  the  sides  to  the  road-bed. 


FIG.  17, 

This  is  the  standard  section  given  for  state  aid  roads 
in  New  York,  using  a  ditch  two  feet  deep  with  side 
slopes  two  to  one.  This  form  is  also  used  by  the 
Massachusetts  Highway  Commission,  who  recommend 
ditches  3  feet  deep,  I  foot  wide  at  bottom  and  with 
slopes  2  to  I.  Fig.  1 8  shows  section  recommended 
by  the  Illinois  Highway  Commission  for  use  in  level 


•25-—  — i K- — -S'- — ->k- 8- 


FIG.  18. 

country,  where  the   roadway  is  formed    by  material 
excavated  from  the  side  ditches. 

On  the  average  country  road,  surface  drainage  will 
be  amply  secured  by  gutters  1 8  inches  to  2  feet  below 
the  crown  of  the  roadway,  and  side  ditches  of  greater 
depth  are  a  source  of  unnecessary  expense.  Where 
under  drainage  is  necessary  it  should  be  accomplished 
by  tile  or  other  covered  drains. 


IMPROVEMENT   OF   COUNTRY  ROADS.  79 

ART.  26.     EARTHWORKS. 

Improvements  to  the  road-bed  of  an  existing  coun- 
try road  may  have  for  their  object  the  reduction  of 
gradient  upon  steep  inclinations  by  cutting  the  material 
from  the  road-bed  and  lowering  the  surface  of  the 
road  on  the  upper  part  of  the  grade,  and  filling  in 
correspondingly  on  the  lower  part,  or  they  may  be 
intended  to  provide  better  drainage  by  raising  the 
road  across  low  ground. 

In  the  construction  of  new  roads,  the  formation  of 
the  road-bed  consists  in  bringing  the  surface  of  the 
ground  to  the  grade  adopted  for  the  road.  This  grade 
should  be  carefulty  established  upon  an  accurate  pro- 
file of  the  line,  in  such  manner  as  to  give  as  little 
earthwork  as  possible,  both  to  render  the  cost  of  con- 
struction low,  and  to  avoid  unnecessarily  marring  the 
appearance  of  the  country  in  vicinity  of  the  road. 
The  most  desirable  position  of  the  grade  line  is  usually 
that  which  makes  the  amounts  of  cut  and  fill  about 
equal  to  each  other,  especially  where  room  for  borrow- 
pits,  or  spoil-banks,  would  be  expensive,  and  it  is 
desirable  to  make  the  embankment  for  the  most  part 
of  the  material  taken  from  the  road  excavations.  On 
side-hill  work,  one  side  of  the  road  is  commonly  in  cut 
and  the  other  in  fill,  and  where  the  side  slopes  are 
steep,  it  is  usually  better  to  make  the  road  mostly 
in  cut  on  account  of  the  difficulty  of  forming  stable 
embankments  on  steep  ground.  In  balancing  cuts  and 
fills,  it  is  necessary  to  estimate  the  quantities  for  the 
full  width,  including  side  ditches,  as  the  grade  should 
be  placed  high  enough  to  permit  using  the  material 
cut  from  the  ditches  in  the  embankment. 

Shrinkage.     Earth,    in   embankment,    will   compact 


80         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

closer  than  it  is  found  in  its  natural  state,  and  allow- 
ance for  shrinkage  must  be  made  in  estimating  the 
amount  of  excavation  necessary  to  form  a  given 
embankment.  On  an  average,  ordinary  soil  may  be 
expected  to  shrink  10  to  12  per  cent  of  its  bulk;  gravel 
or  sand  will  shrink  a  little  less  than  this,  8  or  9  per  cent; 
light  surface  soils  a  little  more,  14  or  15  per  cent. 
The  shrinkage  may  also  be  somewhat  affected  by  the 
method  of  construction  used  in  forming  the  embank- 
ment, being  slightly  less  for  work  placed  by  wagons 
than  for  that  by  scrapers,  and  still  less  for  wheelbarrow 
work. 

Settlement.  In  forming  an  embankment,  allowance 
must  sometimes  be  made  for  subsequent  settlement, 
by  raising  the  top  of  the  embankment  above  the 
required  grade.  Where  scrapers  are  used,  the  earth 
will  usually  be  well  compacted  in  placing,  and  no 
allowance  is  necessary;  with  dump  carts  or  wagons  the 
compacting  is  not  so  thorough,  and  a  small  allowance 
should  be  made;  while  when  wheelbarrows  are  used  or 
the  earth  is  thrown  into  place  with  shovels,  an  allow- 
ance of  10  or  12  per  cent  must  be  added  to  the  height 
of  the  embankment,  in  order  to  allow  for  the  final 
•shrinkage.  Rock  occupies  more  space  in  embank- 
ment than  in  excavation,  and  does  not  need  allowance 
for  shrinkage. 

Embankments.  When  embankments  are  to  be  con- 
structed, brush  and  weeds  should  be  removed  from  the 
site  and  at  points  where  the  filling  is  thin,  it  is  desirable 
to  remove  all  vegetable  matter  and  soft  material,  to 
prevent  unequal  settling  and  the  formation  of  soft  and 
spongy  places  in  the  surface  of  the  road-bed. 

In  constructing  embankments  across  wet  and 
unstable  ground,  it  is  frequently  necessary  to  form  an 


IMPROVEMENT  OF  COUNTRY   ROADS.  8 1 

artificial  foundation  upon  which  to  place  the  earth- 
embankment.  This  may  be  accomplished  in  some 
cases  by  excavating  a  little  of  the  soft  material  and 
substituting  sand  or  gravel,  or  in  other  cases  it  may  be 
advisable  to  employ  layers  of  brushwood  or  fascines 
as  a  support  for  the  enbankment.  Sometimes  it  may 
be  possible  to  drain  the  soft  material  by  deep  ditches, 
so  as  to  render  it  capable  of  sustaining  the  road,  and 
in  all  cases  drainage  should  be  provided  in  so  far  as 
possible  to  make  the  embankment  more  secure. 

When  embankments  are  to  be  found  on  sloping 
ground,  the  surface  of  the  ground  should  be  stepped 
off,  in  order  to  hold  the  earth-filling  from  sliding  upon 
the  natural  surface  at  the  line  of  contact  between  the 
two,  until  it  becomes  sufficiently  settled  for  the  develop- 
ment of  cohesion  to  cause  it  to  become  one  solid  mass. 

In  many  cases  where  roads  are  to  be  constructed 
along  steep  slopes,  it  is  found  cheaper  to  use  retaining 
walls  to  sustain  the  road  upon  the  lower  side  and  the 
earth  cutting  on  the  upper  side  than  to  cut  long  slopes 
or  form  high  embankments. 

Catch-water  drains  are  necessary  on  the  natural  sur- 
face above  the  top  of  all  high  slopes  in  cuttings  to 
prevent  the  surface  water  from  washing  down  and 
destroying  the  face  of  the  slope. 

Where  springs  are  tapped  by  a  cutting,  drains  must 
be  provided  to  remove  the  water  without  injury  to 
the  slope;  and  \vhere  the  subsoil  may  become  wet  in 
rainy  weather,  it  may  be  necessary  to  provide  sub- 
surface drains  along  the  slope  to  prevent  the  earth 
becoming  saturated  and  sliding  down  into  the  roadwaj^. 

Slopes,  both  of  excavation  and  embankment,  are 
greatly  improved  by  being  sodded  or  sown  with  grass. 
This  aids  in  the  maintenance  of  the  slopes,  by  render- 


82          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ing  them  more  capable  of  resisting  the  abrading  action 
of  such  water  as  falls  upon  them.  It  also  greatly 
improves  their  appearance. 

The  most  important  principle  involved  in  the  forma- 
tion of  a  road-bed,  which  should  be  always  in  mind,  is 
that  earth,  in  order  either  to  sustain  a  load  or  to  main- 
tain a  slope,  must  be  kept  dry,  or  at  least  prevented 
from  becoming  saturated  with  water,  as  both  the 
cohesive  and  frictional  resistances  of  earth  are  dimin- 
ished or  destroyed  when  it  becomes  wet,  and  it  is  also 
then  liable  to  the  disturbing  action  of  frost. 

Methods  of  Handling  Earth.  In  the  grading  of  roads 
or  streets,  the  earth  is  commonly  moved  by  scrapers, 
or  wagons,  after  being  loosened  by  plowing.  For 
ordinary  work  the  common  railroad  plow  is  used,  drawn 
by  two,  or  in  hard  material  four,  horses.  In  breaking 
up  very  hard  material,  like  an  old  gravel  surface,  a 
rooter  plow  may  be  needed  with  four  or  six  horses. 
Economical  handling  of  the  material  requires  that  it 
be  well  loosened  and  the  plowing  is  usually  but  a  small 
part  of  the  cost. 

For  moving  the  loosened  earth,  drag  scrapers  may 
be  used  for  short  hauls;  they  are  economical  for 
distances  up  to  about  100  feet.  For  distances  greater 
than  about  80  to  I  oo  feet  wheel  scrapers  will  be  more 
economical;  for  the  shorter  hauls,  the  small  (number  l) 
scraper,  with  a  single  team  to  handle  each  scraper; 
for  longer  hauls,  above  200  to  300  feet,  the  larger 
(number  3)  scrapers,  with  snatch  teams  to  load  them. 
For  hauls  greater' than  about  500  to  600  feet,  wagons, 
loaded  by  men  with  shovels,  will  usually  be  cheaper 
than  scraper  work. 

In  flat  country  where  the  grades  conform  closely  to 
the  natural  surface,  and  the  road-bed  is  formed  with 


IMPROVEMENT   OF  COUNTRY  ROADS.  83 

earth  taken  from  the  side  ditches,  the  use  of  the  eleva- 
ting grader  is  usually  economical  and  frequently  makes 
possible  the  construction  of  the  road  at  very  low  cost. 
This  consists  of  a  frame  resting  upon  four  wheels, 
from  which  is  suspended  a  plow  and  a  wide  traveling 
belt.  The  plow  loosens  the  earth  and  throws  it  upon 
the  inclined  belt,  which  carries  it  to  one  side  and  de- 
posits it  near  the  middle  of  the  road.  The  ordinary 
machines  are  built  to  deliver  the  material  at  about  14 
and  17  feet  horizontally  from  the  point  at  which  it 
is  excavated.  They  are  usually  operated  with  eight 
horses. 

The  ordinary  road  machine,  or  scraping  grader,  is 
also  a  convenient  tool  for  this  kind  of  work,  and  when 
the  amount  of  material  to  be  moved  is  small,  and  the 
work  consists  in  cutting  shallow  side  ditches  and  form- 
ing a  road  surface  with  material  taken  from  them,  is 
usually  the  most  economical  tool  to  emplo3\ 

Work  done  by  scrapers  will  usually  be  left  in  rough 
and  lumpy  condition.  For  smoothing  the  surface, 
after  the  earthwork  is  roughly  completed,  the  scraping 
grader  or  some  form  of  road  leveler  ma}^  be  used.  For 
this  purpose  the  blade  is  set  so  as  to  cut  off  the  tops 
of  the  ridges  and  lumps,  and  fill  up  the  hollows,  with- 
out carrying  along  any  earth. 

Work  Required  in  Moving  Earth.  The  work 
required  for  moving  earth  under  approximately  the 
same  conditions  differs  widely  in  practice.  It  depends 
upon  the  character  of  the  material,  the  methods 
adopted  for  the  work,  the  kind  of  labor  available, 
and,  most  important  of  all,  the  skill  with  which  it  is 
managed. 

Loosening.  In  ordinary  compact  soil  a  plow  and 
team  with  driver  and  plow  holder  will  loosen  30  to  40 


84         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

cubic  yards  per  hour.  If  the  material  be  very  hard, 
an  extra  team  and  man  will  be  required  for  about  the 
same,  or  a  little  less,  amount  of  earth;  while  in  hard 
buckshot,  or  in  breaking  up  an  old  road  surface,  not 
more  than  one-half  the  amount  may  be  loosened. 

Drag  Scrapers.  On  short  hauls  up  to  about  50 
feet,  a  team  with  drag  scraper  may  handle  from  about 
4  to  7  cubic  yards  per  hour,  and  one  man  will  load 
for  2  or  3  scrapers,  depending  upon  the  distance. 
If  the  lead  be  1 00  feet,  the  drag  scraper  should  move 
from  3  to  5  cubic  yards  per  hour,  and  one  man  should 
load  about  4  scrapers. 

Wheel  Scrapers.  Small  (number  i)  wheel  scrapers 
have  a  capacity  of  about  £  cubic  yard  of  compacted 
earth,  and  for  a  haul  of  100  feet  may  be  expected  to 
give  about  the  same  results  as  drag  scrapers.  For 
longer  hauls,  about  one  minute  of  time  of  team  and 
driver  will  be  required  per  trip  for  each  1 00  feet  of 
additional  distance,  or  about  5  minutes  for  each  yard 
of  material  moved. 

The  large  size  (number  3)  wheel  scraper  may  be  con- 
sidered as  carrying  J  cubic  yard  at  a  trip.  A  snatch 
team  and  extra  man,  or  two  extra  men,  will  be  required 
to  load.  These  will  load  a  scraper  in  an  average  of 
from  one  minute  to  two  minutes.  Two  or  three  min- 
utes may  be  allowed  for  loss  of  time  of  scrapers  on  each 
trip  in  loading  and  unloading,  and  one  minute  for 
each  100  feet  of  haul.  Thus,  with  a  lead  of  300  feet, 
a  trip  would  be  made  in  five  or  six  minutes;  10  or  12 
trips  per  hour,  or  from  3^  to  4  cubic  yards  per  hour 
for  each  scraper. 

Wagons.  Over  ordinary  earth  roads  a  team  and 
wagon  will  carry  an  average  load  of  I  cubic  yard;  on 
good  hard  earth  roads  1}  yards  may  be  taken.  In 


IMPROVEMENT  OF  COUNTRY  ROADS.      85 

loading  ordinary  soil  which  has  been  loosened  by  plows, 
men  may  be  expected  to  average  from  li  to  2  cubic 
yards  per  hour.  When  the  work  is  fairly  well  organized 
and  as  many  men  are  employed  in  loading  as  can  con- 
veniently work  about  a  wagon  (usually  about  7  or  8) 
the  loss  of  time  of  each  team  in  loading,  unloading,  etc. 
may  average  about  5  to  7  minutes  for  loads  of  one  cubic 
yard,  while  the  time  occupied  in  hauling  will  average 
about  one  minute  for  each  1 00  feet  of  lead.  When  dump 
wagons  are  used,  about  one  minute  would  be  saved 
on  each  trip. 

In  estimating  the  cost  of  earthwork,  about  20  to  25 
per  cent  should  be  added  to  the  labor  cost  for  con- 
tractor's profit,  contingencies,  etc.  The  skill  with 
which  ea^rthwork  is  managed  has  much  to  do  with  the 
cost.  Failure  to  properly  organize  and  systematize 
the  work  may  easily  increase  the  labor  cost  50  per 
cent.  It  is  not  uncommon  to  find  that  two  pieces  of 
work  identical  in  character,  and  conducted  under  the 
same  conditions,  differ  25  per  cent  in  cost,  because  of 
the  difference  in  the  foremen  handling  the  work. 

ART.  27.     EARTH  ROADS. 

The  maintenance  of  an  earth  road  surface  in  good 
condition  consists  in  keeping  it  crowned  and  smoothed, 
so  that  water  which  falls  upon  the  surface  flows  away 
immediately  into  the  gutters  without  remaining  upon 
the  road  long  enough  to  do  serious  harm  in  softening  it. 
If  ruts  and  depressions  are  allowed  to  form  in  the  road 
surface,  they  will  hold  water  until  it  is  absorbed  into 
the  road  or  evaporated,  thus  softening  the  road  so 
that  wheels  will  cut  deeply  into  it,  and  gradually 
destroy  its  firmness. 


86          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

The  improvement  of  an  earth  road  surface  which  is 
not  in  good  condition  must,  therefore,  be  effected 
mainly  by  reshaping  it  into  a  form  with  proper  crown 
to  shedthe  water.  Its  subsequent  maintenance  requires 
that  it  be  frequently  smoothed  to  prevent  the  forma- 
tion of  ruts.  It  is  practically  impossible  to  maintain 
an  ordinary  earth  road  in  good  condition  by  the 
method  of  annual  repairs.  Where  this  method  is 
followed,  the  road  is  usually  shaped  up  with  a  road 
grader,  after  it  dries  sufficiently  in  the  spring,  and 
may  present  a  good  surface  during  the  summer  and 
fall.  It  will,  however,  be  worn  hollow  by  the  time 
bad  weather  sets  in,  and  will  be  in  condition  to  hold 
water  and  become  saturated  by  heavy  rains  or  melting 
snow. 

Shaping  Section.  The  form  of  cross  section  which 
should  be  used  has  already  been  discussed  in  Art.  25, 
and  the  drainage  of  the  road  should  be  provided  for 
\vhere  necessary  as  described  in  Chapter  II.  For 
cleaning  the  side  ditches  and  forming  the  surface  of 
the  road,  the  ordinary  road  grader,  or  scraping  grader, 
is  used  as  mentioned  in  Art.  26.  In  this  work,  the 
blade  of  the  grader  is  set  so  as  to  carry  the  material 
from  the  ditches  toward  the  middle  of  the  road,  and 
repeated  trips  are  made  until  the  proper  crown  is  given 
to  the  road  surface.  In  doing  this,  care  should  be 
taken  not  to  leave  a  ridge  of  soft  material:  at  the  middle 
of  the  road,  but  to  spread  it  evenly  so  that  travel  may 
take  any  part  of  the  road  surface,  and  thus  compact  it 
evenly.  This  may  be  accomplished  by  slightly  raising 
the  end  of  the  blade  of  the  grader  nearest  the  middle 
of  the  road.  In  using  the  grader,  the  amount  of 
material  moved  and  its  distribution  are  controlled  by 
changing  the  angle  at  which  the  blade  is  set,  and  the 


IMPROVEMENT  OF  COUNTRY  ROADS.       8/ 

elevation  of  its  ends.  Experience  in  handling  the 
machine  is  necessary  to  its  skillful  use,  and  the  amount 
of  work  required  in  forming  a  road  is  largely  dependent 
upon  the  skill  and  experience  of  the  man  operating 
the  machine.  Good  results  in  such  work  also  require 
that  the  teams  used  be  well  broken  to  the  work. 

Where  roads  are  shaped  in  this  manner  in  the  spring, 
the  work  should  be  done  before  the  surface  has  become 
dry  and  hard,  and  while  the  earth  is  in  condition  to 
pack  and  unite  with  the  surface  upon  which  it  is  placed. 
After  the  ground  has  become  dry  and  hard,  the  work  is 
more  difficult  and  expensive,  and  the  road  is  usually 
left  in  bad  condition  because  the  material  moved, 
being  hard  and  lumpy,  does  not  pack  readil3T  under 
travel. 

Smoothing  Surface.  The  maintenance  of  an  earth 
road  in  good  condition  requires  that  surface  be  fre- 
quently smoothed  so  as  to  prevent  the  formation  of 
ruts,  which  may  hold  water  when  rain  comes.  Repair-- 
ing the  road  by  reforming  the  surface  when  it  has 
gotten  out  of  shape  may  improve  it  so  that  it  will 
remain  in  fair  condition  so  long  as  weather  conditions 
are  favorable,  but  when  rain  comes  the  surface  will  be 
softened  so  that  wheels  cut  in  to  a  small  depth,  making 
small  indentations.  These,  if  allowed  to  remain,  will 
hold  water  at  the  next  rain,  causing  the  road  to  become 
soft  to  a  greater  depth  and  deeper  ruts  to  form.  If, 
however,  after  each  rain  the  road  be  smoothed  out, 
eliminating  the  ruts,  and  moving  a  little  earth  toward 
the  middle  of  the  road  to  replace  that  lost  through 
wear,  the  road  surface  will  be  hardened  and  improved 
at  each  treatment,  and  will  not  retain  water  when  con- 
tinued rains  come  upon  it. 

The  smoothing  of  the  road  surface  should  be  done 


88         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

when  the  road  is  drying  out  after  a  rain;  when  it  is 
not  too  muddy,  but  before  it  has  become  hard.  The 
earth  is  then  in  condition  to  pack  readily  under  travel, 
and  will  form  a  smooth  hard  surface  when  it  becomes 
dry.  If  undertaken  when  the  surface  is  too  wet,  it 
may  be  muddy  and  sticky  to  work;  after  it  becomes 
dry,  no  good  can  be  accomplished  by  working  it,  as  it 
will  not  pack  smooth  and  hard.  When  a  road  is  kept 
shaped  up  by  smoothing  it  after  each  rain,  the  earth 
composing  the  road  surface  becomes  puddled,  through 
being  worked  while  wet,  until  it  becomes  practically 
impervious  to  water  and  forms  a  very  hard  crust  on  the 
surface.  This  effect  is  observed  upon  all  soils  which 
soften  upon  absorbing  water,  and  become  hard  when 
dry,  out  is  most  noticeable  upon  clay  or  other  heavy 
soils.  The  soil  which  makes  the  worst  and  most 
sticky  mud  when  allowed  to  become  saturated  with 
water  makes  the  hardest  and  most  impervious  surface 
when  well  maintained. 

Methods  for  Smoothing  Surface.  Several  methods 
have  .sometimes  been  employed  for  smoothing  the 
surface  of  an  earth  road.  For  the  purpose  of  smoothing 
out  the  ruts  in  the  spring,  when  a  muddy  road  is  drying 
up,  a  railroad  rail  12  to  1 6  feet  in  length  has  sometimes 
been  used,  the  rail  being  drawn  by  teams  hitched  at 
the  ends  so  as  to  cut  off  the  ridges  and  fill  the  ruts.  A 
heavy  stick  of  timber  faced  with  steel  on  one  edge  has 
also  been  used  in  the  same  way.  These  methods  may 
prove  quite  efficient  at  times  when  the  roads  are  in  bad 
condition,  causing  the  surface  to  dry  smoother  than 
would  otherwise  be  the  case. 

The  scraping  grader  is  frequently  used  for  light 
trimming  of  the  surface,  but  is  not  usually  an  economical 
tool  to  use  unless  heavier  \york  is  to  be  done,  on  account 


IMPROVEMENT  OF  COUNTRY  ROADS.      89 

of  the  weight  of  the  machine  and  the  cost  of  operating  it. 
Several  types  of  light  road  scrapers,  or  road  levelers, 
as  they  are  usually  called,  requiring  only  a  single  team 
and  driver,  or  perhaps  also  a  man  to  operate  the 
machine,  are  occasionally  used  for  this  purpose.  These 
levelers  are  sometimes  mounted  upon  two  wheels,  and 
the  blade  made  adjustable  in  position;  others  are  simply 
cutting  blades  which  slide  upon  the  ground  in  fixed 
position.  They  frequently  do  good  work  in  smoothing 
the  surface  when  the  soil  is  in  proper  condition, 
although  they  do  not  pack  the  material  upon  the 
surface  of  the  road. 

Road  Drqg.  The  cheapest  and  most  successful 
method  yet  devised  for  maintaining  the  surface  of  a 
road  in  good  condition  is  by  the  use  of  the  road  drag. 
This  method  has  been  used  with  great  success  in  the 


FIG.  19. 

states  of  the  Mississippi  valley,  where  the  maintenance 
of  earth  roads  in  condition  to  be  used  at  all  seasons 
had  previously  been  considered  an  almost  hopeless  task. 
Its  introduction  is  largely  due  to  the  efforts  of  Mr. 
D.  Ward  King,  of  Maitland,  Mo.,  who  spent  most  of 
his  time  for  a  number  of  years  in  introducing  and 
explaining  his  method  of  using  the  "split  log  drag." 
Fig.  19  shows  a  drag  as  commonly  constructed  of  split 


go         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

logs.  The  following  description  of  the  drag  and  its 
operation  is  taken  from  the  report  of  the  Illinois 
Highway  Commission  for  1906: 

"The  log  should  be  from  10  to  12  inches  in  diameter 
and  about  9  feet  long.  The  holes  in  the  front  half  of 
the  log  should  be  bored  so  that  a  slight  slant  forward  is 
given  to  the  lower  part  of  the  front  face  of  the  split  log. 
The  holes  in  the  rear  log  are  bored  so  that  its  flat  face 
will  be  perpendicular  to  the  sticks  forming  the  con- 
necting braces  which  should  be  tapered  at  the  ends 
so  that  they  will  fit  snugly  into  the  holes  bored  into  the 
logs.  The  holes  should  not  be  less  than  two  inches  in 
diameter.  The  ends  of  the  cross  sticks  should  be 
split  and  wedges  driven  so  as  to  secure  the  cross  braces 
in  place.  The  wedges  should  be  driven  crosswise  of 
the  grain  of  the  log  or  plank  so  as  not  to  split  it.  A 
diagonal  cross  brace  is  placed  between  the  logs  at  the 
leading  end  to  stiffen  the  frame  of  the  drag.  The 
distance  from  the  face  of  the  back  log  to  the  face  of  the 
front  log  should  be  about  three  feet.  The  lower  front 
edge  or  toe  of  the  drag  should  be  protected  by  a  strip 
of  old  wagon  tire,  or  other  piece  of  iron,  about  a  quarter 
of  an  inch  thick  and  3  or  4  inches  wide  and  about  4 
feet  long.  This  strip  of  iron  should  be  bolted  to  the 
front  log  and  the  heads  of  the  bolts  countersunk.  The 
strip  of  iron  should  not  be  carried  the  entire  length  of 
the  front  log. 

"Chains  should  be  provided  with  which  to  haul  the 
drag,  arranged  with  a  short  and  long  hitch  as  shown  in 
the  sketch,  so  that  the  drag  will  travel  at  an  angle  of 
about  45  degrees  with  the  direction  of  the  road.  It 
will  be  noticed  from  the  sketch  that  the  long  hitch  of 
the  chain  goes  over  the  log  around  one  of  the  cross 
pieces  rather  than  through  a  hole  in  the  front  log. 


IMPROVEMENT   OF   COUNTRY  ROADS.  91 

This  allows  the  earth  to  slide  unobstructedly  along  the 
front  face  of  the  drag. " 

The  drag  may  also  be  made  of  planks,  instead  of 
logs;  2  by  12  inch  planks  are  used  for  this  purpose, 
reinforced  on  the  inner  side  by  2  inch  by  6  inch  strips, 
to  provide  a  greater  thickness  of  wood  through  which 
to  bore  the  holes. 

"  When  the  road  drag  is  properly  used  it  spreads  out 
the  layer  of  impervious  soil  over  the  surface  of  the  road, 
filling  up  the  ruts  and  hollows  until  a  smooth  surface  is 
secured.  As  a  small  amount  of  material  is  always  to  be 
pushed  to  the  center,  a  slightly  rounded  effect  will  be 
given  to  the  road,  which  may  be  increased  or  decreased 
as  desired  by  subsequent  dragging.  By  forcing  the 
mud  into  the  hollows  and  ruts,  it  is  evident  that  the 
water  must  go  out,  which  it  does  by  running  off  to  the 
side  of  the  road.  The  drying  out  of  the  road  is  thus 
much  facilitated  and  the  road  is  made  immediately 
firmer  because  the  water  is  squeezed  out. 

"The  effect  of  traffic  over  the  road  tends  to  press 
down  and  thoroughly  compact  the  top  of  the  road  and 
each  thin  layer  of  puddled  earth  which  the  drag 
spreads  over  the  surface  every  time  it  is  used.  After 
the  first  few  draggings  it  will  be  noticed  that  the  road  is 
becoming  constantly  smoother  and  harder  so  that  the 
effect  of  a  rain  is  scarcely  noticeable,  the  water  running 
off  the  smooth  hard  surface  which  absorbs  but  little  of 
it." 

The  action  of  the  drag  differs  from  that  of  an  ordi- 
nary scraper  or  leveler  in  that  it  packs  the  material 
upon  the  surface,  while  the  leveler  merely  smooths  the 
road  by  trimming  off  the  high  places  and  distributing 
the  material  into  the  low  places. 

Constant    attention    is    necessary    to    maintain    an 


92         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

earth  road  in  good  condition,  The  dragging  should 
be  done  as  soon  after  each  rain  as  possible,  and  at  all 
seasons  of  the  year.  If  dragging  is  done  before  a 
cold  spell,  the  road  will  freeze  in  a  smooth  conditioii, 
and  will  be  in  good  condition  when  the  frost  leaves  it. 

The  amount  of  work  required  in  dragging  roads  is 
comparatively  small.  While  frequent  attention  is 
necessary,  the  work  to  be  done  at  one  time  is  insignifi- 
cant. In  several  instances  it  has  been  found  that  the 
cost  of  maintenance  by  dragging  is  much  less  than  the 
expenditure  previously  incurred  for  shaping  up  the 
road  with  the  grader  in  the  spring. 


SAND    ROADS. 

When  a  road  surface  is  composed  of  sand,  it  will  be 
more  firm  when  the  sand  is  damp  and  more  unstable 
in  dry  weather.  Such  roads  -require  very  different 
treatment  from  those  on  clay  or  loam.  No  attempt 
should  be  made  to  round  up  the  road,  as  it  is  an  advan- 
tage to  retain  as  much  moisture  as  possible.  If  clay 
is  available,  it  is  desirable  to  mix  clay  with  sand  in 
the  road  surface.  If  clay  is  put  upon  the  surface  when 
the  materials  are  damp,  the  traffic  will  mix  them 
thoroughly  together  and  the  surface  will  become  hard 
when  it  dries  out,  and  make  a  good  road  surface.  It  is 
only  necessary  to  keep  the  surface  smooth  while  it 
is  drying. 

As  a  general  thing,  unless  a  sand  road  can  be  resur- 
faced, it  is  better  to  avoid  disturbing  it,  and  the  less 
work  done  upon  it  the  better.  If  there  is  but  little 
travel  over  it,  sod  and  weeds  may  grow  in  the  sand, 
and  these  should  not  be  eradicated. 


IMPROVEMENT  OF  COUNTRY  ROADS.      93 

ART.  28.     GRAVEL  ROADS. 

In  the  improvement  of  a  country  road,  where  the 
construction  of  a  good  Telford  or  macadam  road  can- 
not be  undertaken,  a  surface  of  gravel  may  frequently 
be  used  to  advantage,  giving  much  better  results  than 
could  be  obtained  with  the  surface  of  earth.  Even 
a  light  layer  of  gravel  may  frequently  prove  of  very 
great  benefit. 

Where  the  subsoil  is  of  a  porous  nature  and  well 
drained,  a  layer  of  three  or  four  inches  of  gravel,  or 
sometimes  even  less,  well  compacted,  will  constitute 
a  very  considerable  improvement,  especially  if,  as  is 
usual  with  these  light  soils,  the  nature  of  the  mate- 
rial of  the  road-bed  is  particularly  unsuitable  for  the 
wearing-surface,  difficult  to  compact  sufficiently  to 
shed  water,  and  likely  to  become  soft  when  wet. 

Gravel  for  use  on  roads  should  be  of  hard,  tough 
material,  capable  of  resisting  the  abrasion  of  traffic. 
Natural  gravels  may  differ  widely  in  the  character  of 
the  materials  composing  them,  and  in  many  instances 
are  harder  and  more  durable  than  the  native  stone  of 
the  same  locality.  Nearly  any  gravel  will  be  an 
improvement  upon  an  ordinary  road  surface,  but 
where  an  important  road  is  being  improved  the  material 
should  be  carefully  selected.  The  size  of  pebbles 
composing  the  gravel  is  important  in  considering  its 
value  for  road  purposes.  As  a  general  thing  they 
should  not  be  more  than  I  inch,  or  at  most  I?  inches, 
in  greatest  dimension.  The  size  should  not  be  too 
uniform,  but  the  gravel  should  contain  enough  small 
fragments  to  fill  the  interstices  between  the  larger 
pieces,  in  order  that  it  may  pack  well  in  the  road. 
When  the  gravel  is  too  fine  or  too  uniform,  it  will  not 


94         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

bond  properly,  and  will  be  difficult  to  compact  into  a 
hard  surface.  The  proper  gradation  of  sizes  is  the 
most  important  characteristic  of  good  gravel.  The 
larger  pieces  are  usually  the  hardest  and  most  durable 
part  of  the  gravel.  They  have  resisted  the  grinding 
action  which  has  reduced  the  other  material  to  smaller 
fragments.  It  is  desirable  therefore  that  there  be 
only  enough  fine  material  to  fill  the  interstices  in  that 
of  larger  size.  When  fine  material  is  in  considerable 
excess  the  gravel  should  be  screened  in  order  to  get  the 
best  results.  In  many  instances,  it  is  possible  to 
greatly  improve  the  quality  of  gravel  by  screening 
into  two  or  three  sizes  and  then  recombining  these  in 
proper  proportions  to  produce  the  most  dense  material. 

Binder.  In  order  to  bind  well  in  the  road  surface, 
the  small  spaces  between  the  fragments  of  gravel  must 
be  filled  with  fine  material.  Without  this  the  frag- 
ments composing  the  gravel  will  roll  upon  each  other 
and  not  pack  well.  Natural  gravel  may  contain 
enough  fine  material  or  soft  material  which  will  crush 
under  the  loads  coming  upon  it  to  cause  it  to  bind 
well  in  the  road;  or  it  may  be  necessary  to  add  some 
material  to  the  gravel  surface  to  act  as  a  binder. 
Clay,  loam,  or  stone  screenings  may  be  used  as  a  binder. 
It  is  desirable  to  use  as  little  binder  as  is  consistent 
with  the  proper  bonding  of  the  gravel.  When  in  excess 
it  has  a  tendency  to  cause  the  road  to  soften  in  wet 
weather  and  to  crack  in  dry  weather.  This  is  espe- 
cially noticeable  with  clay  binder.  If  gravel  contains 
too  much  fine  material,  or  when  the  fine  material  is 
unevenly  distributed  through  the  gravel,  it  should  be 
passed  over  a  one  half  inch  screen,  and  the  fine  part 
thus  removed  be  used  on  the  surface  as  a  binder. 

When  gravel  contains  considerable  large  material, 


IMPROVEMENT   OF   COUNTRY  ROADS.  95 

a  screen  of  i^  to  i^  inches  mesh  may  be  used  to  remove 
such  material  from  the  portion  of  the  gravel  to  be  used 
in  the  surface  layer  of  the  road.  If  the  road  is  to  be 
sufficiently  thick  to  be  constructed  in  two  layers, 
the  larger  pebbles  screened  from  the  gravel  will  be 
suitable  for  use  in  the  lower  course. 

Construction.  In  'the  construction  of  a  road  with 
gravel  surface  the  road-bed  should  first  be  brought  to 
the  proper  grade ,ywith  a  form  of  cross  section  the  same 
as  that  to  be  given  the  finished  road.  The  gravel  is 
then  placed  upon  it  and  rolled  to  a  surface,  or  left  to 
be  compacted  by  the  traffic.  It  is  always  advantageous 
when  possible  to  compact  the  road  by  rolling.  The 
road-bed  should  be  well  rolled  before  placing  the 
gravel,  and  the  gravel  surface  afterward.  A  smooth 
hard  surface  may  thus  be  produced,  upon  which  the 
wheels  of  loaded  vehicles  may  roll  without  producing 
any  visible  impression. 

In  preparing  a  road-bed  for  gravel  surface,  when  a 
light  coating  of  gravel  is  to  be  used,  the  surface  of  the 
ground  is  shaped  up  with  the  grader  in  the  ordinan^ 
manner,  but  using  a  section  flatter  than  the  finished 
road  is  to  have.  The  gravel  is  then  placed  and  spread 
so  as  to  have  the  proper  thickness  at  the  middle  and 
diminish  the  thin  edges  at  the  sides  of  the  road.  On 
a  good  well  drained  road-bed,  this  construction  with 
gravel  four  or  five  inches  thick  at  the  middle  of  the 
road  ma}^  make  a  very  good  country  road.  Good 
drainage  is,  however,  essential  to  success  with  such  a 
road. 

On  important  roads  gravel  is  often  used  instead  of 
broken  stone,  in  the  manner  described  in  Chapter  V. 
In  many  localities,  gravel  exists  which  is  superior  in 
hardness  and  durability  to  the  local  stone  available 


96         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

for  road  metal.  In  such  instances  gravel  is  often 
used  for  surfacing  a  stone  road. 

Maintenance.  A  new  gravel  road  when  first  opened 
to  travel  may  require  considerable  attention  to  keep 
it  in  good  condition.  The  surface  should  be  watched 
and  all  ruts  or  depressions  which  may  form  be  at  once 
filled  up.  When  new  material  must  be  added  'in 
repairing  a  gravel  surface,  it  should  be  fine  and  contain 
more  binding  material  than  the  gravel  used  in  the 
first  construction  of  the  road. 

After  a  gravel  road  is  thoroughly  compacted  by  the 
traffic,  less  attention  is  required  to  keep  it  in  surface 
until  it  is  worn  thin  enough  to  require  resurfacing. 

ART.  29.     OILED  ROADS. 

The  use  of  oil  for  hardening  the  surfaces  of  earth 
and  gravel  roads  originated  in  California,  where  it 
has  rapidly  extended  into  common  practice.  The  use 
of  oil  was  at  first  intended  only  for  the  purpose  of 
laying  the  dust,  and  the  surface  of  the  road  was 
sprinkled  two  or  three  times  during  the  summer  with 
a  light  coating  of  oil.  The  effect  of  the  oil  upon  the 
road  was  such  as  to  very  quickly  modify  both  the 
purpose  and  the  method  of  the  application,  and  many 
roads  were  soon  constructed  in  which  oil  was  used  for 
the  purpose  of  binding  together  the  material  of  the 
road  surface,  and  thus  forming  a  crust  over  the  road 
which  would  take  the  wear  of  traffic.  The  results  in 
general  were  satisfactory,  giving,  in  many  instances, 
smooth  firm  road  surfaces,  free  from  dust  during  the 
summer,  and  without  mud  in  winter. 

Methods  0}  Construction.  Several  methods  have  been 
employed  in  the  construction  of  oiled  roads.  In  the 


IMPROVEMENT  OF  COUNTRY  ROADS.      97 

earlier  construction,  the  oil  was  applied  to  the  road 
when  hard  and  smooth,  the  surface  being  sprinkled 
with  the  oil,  which  was  absorbed  into  the  surface. 
The  following  extract  from  a  paper  by  T.  F.  White, 
in  Engineering  Record  for  Feb.  22,  1902,  explains  this 
method  as  commonly  practiced: 

"Oil  on  roads,  besides  aiding  to  make  a  wearing 
surface,  preserves  the  road-bed.  It  follows  therefore, 
that  the  road-bed  should  be  carefully  prepared,  well 
graded  and  shaped,  and  the  surface  smoothed  and 
packed  as  firmly  as  the  material  of  which  it  is  composed 
will  permit  before  the  oil  is  applied.  We  therefore  do 
our  grading  during  the  early  part  of  winter,  that  the 
road-bed  may  have  the  benefit  of  the  winter  rains, 
and  become  packed  from  travel  as  well  as  from  thor- 
ough rolling.  We  roll  after  it  has  become  moistened 
through.  Then  in  the  spring,  while  still  moist,  we  go 
over  it  with  a  blade  grader  or  smoother  or  both,  and 
dress  up  the  surface,  crowning  it  as  desired;  and  as 
soon  thereafter  as  the  surface  is  dry  and  the  weather 
is  settled  and  warm,  the  oil  is  applied,  as  much  in 
quantity  as  the  material  will  absorb  and  mix  with. 
This  has  reference  to  a  road  never  oiled  before. 

"It  may  be  desired  to  put  oil  on  a  road  that  is  not 
in  very  good  shape  as  to  grade  and  smoothness  of 
surface.  It  is  not  recommended  to  apply  oil  to  such 
a  road,  but  circumstances  may  make  it  seem  desirable. 
In  such  a  road  there  may  be  chuck  holes  full  of  dust. 
To  oil  it  we  go  over  the  holes  first,  scraping  out  the  dust, 
filling  them  nearly  full  of  oil,  and  then  with  hoe  and 
rake,  work  in  the  dust,  together  with  sharp  sand  and 
fine  gravel,  which  are  thrown  in  from  a  wagon  drawn 
alongside,  until  the  holes  are  filled  from  bottom  up 
with  oil,  dirt,  and  gravel,  thoroughly  mixed  together. 


98          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

When  all  the  holes  are  filled,  we  apply  a  coat  of  oil  to 
the  whole  surface  of  the  road.  Should  the  road  be 
very  uneven,  however,  and  full  of  holes,  we  prefer  to 
haul  on  gravel,  of  a  kind  that  will  pack,  and  fill  up  the 
holes  and  uneven  places,  saturate  with  water  and  roll 
before  applying  oil.  Should  the  surface  of  a  road  be 
worked  up  to  a  considerable  depth  of  dust,  if  it  is  of  a 
nature  to  pack  with  water  and  compression,  we  drench 
it  thoroughly  and  roll.  But  if  it  will  not  so  pack, 
being  of  too  sandy  a  nature,  we  pour  on  the  oil, 
attempting  to  saturate  all  loose  material  to  the  firmer 
stratum  below.  The  only  rule  we  have  as  to  quantity 
of  oil  to  be  applied  is  to  put  on  sufficient  to  saturate 
all  of  the  loose  covering  of  the  road,  and  secure  some 
penetration  into  the  firm  road-bed  beneath. 

"After  the  oil  is  put  on  in  any  of  these  instances 
some  appliance  for  mixing  the  oil  and  loose  road 
materials  is  run  over  the  surface  backwards  and  for- 
wards until  a  thorough  mixing  is  accomplished.  If 
the  road  surface  is  very  loose,  a  common  steel  lever 
harrow,  with  the  teeth  slanted  back,  is  useful.  This 
may  be  dragged  to  and  fro  longitudinally  along  the 
road,  and  back  and  forth  spirally  across  the  road, 
until  a  thorough  mixing  is  secured.  On  firmer  roads 
and  where  there  is  little  loose  covering,  a  lighter 
implement,  with  numerous  dragging  fingers  suspended 
from  an  axle,  is  better. 

"All  this  has  reference  mainly  to  roads  that  have 
never  been  oiled  before.  When  it  comes  to  oiling  a 
road  the  second,  third,  and  later  seasons,  the  operation 
is  somewhat  different.  Should  the  oiled  surface  be 
cut  through  and  chuck  holes  formed  (but  there  will 
be  very  few  holes  if  the  road  has  been  properly  looked 
after),  we  go  over  these  in  the  manner  previously 


IMPROVEMENT   OF   COUNTRY   ROADS.  99 

noted  for  repairing  chuck  holes,  and  then  apply  a 
dressing  of  oil  to  the  whole  surface,  just  enough 
to  saturate  the  loose  material  and  secure  a  very  slight 
penetration  into  the  old  oiled  surface.  Here  I  will 
call  attention  to  a  danger  we  may  fall  into,  that  of 
putting  too  much  oil  on  the  smooth,  hard  oiled  surface 
we  have  previously  obtained,  softening  it,  and  putting 
it  in  condition  to  rut,  especially  under  heavy  loads. 
We  may  in  this  way  lose  a  part  of  the  results  of  the 
previous  year's  work.  I  made  this  mistake  on  a  road 
last  summer,  so  can  speak  from  experience.  But 
enough  oil  should  be  put  on  to  cover  the  entire  surface 
as  with  a  thin  sheet.  Then  there  will  be  a  surplus  of 
oil,  and  the  road,  if  left  without  further  attention, 
would  be  sticky  and  very  unpleasant  to  travel  over 
for  a  considerable  time  after  the  application.  We 
therefore  follow  this  application  on  smooth  hard  roads 
that  have  previously  been  oiled,  with  a  sprinkling  of 
sand,  using  fine  gravel  and  sharp  sand,  such  as  builders 
use  in  their  mortars.  This  takes  up  the  surplus  oil 
and  adds  to  the  wearing  surface,  and  renders  the  road 
at  once  comfortable  to  travel  over.  The  sand  soon 
becomes  incorporated  with  the  rest  of  the  road  material 
and  packs  down  smooth  and  hard.  The  quantity  of 
sand  put  on  is  just  sufficient  to  take  up  the  surplus 
oil  and  no  more. 

"We  frequently  use  this  sanding  process  also  when 
applying  oil  for  the  first  time  to  a  hard  smooth  road. 
We  have  used  it  on  a  macadamized  road  in  which  the 
surface  was  too  tight  to  absorb  the  oil,  and  obtained 
excellent  results.  It  is  useful  alsq,  where  oil  is  applied 
to  a  tight  adobe  or  other  clay  road.  With  the  oil  and 
sand  a  wearing  surface  may  be  built  up  on  the  clay 
and  be  made  to  last,  while  without  the  sand  the  oil 


IOO       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

has  a  tendency  to  ball  up  with  the  clay  dust  and  carry 
off." 

Oil  applied  to  a  road  surface  in  this  manner  is  ab- 
sorbed by  the  material  of  the  road  covering  to  a  small 
depth,  varying,  according  to  the  character  of  the 
material,  from  about  one  half  inch  to  li  inches.  This 
forms  a  thin  coating  of  oiled  material  over  the  surface 
of  the  road,  which  prevents  the  formation  of  dust  and 
assists  in  preventing  water  from  penetrating  into  the 
road  when  rain  comes  upon  it. 

In  order  to  secure  a  greater  penetration  of  oil,  in 
some  localities,  the  soil  of  the  road  is  loosened  by  the 
use  of  a  harrow  to  a  depth  of  about  two  inches,  before 
the  application  of  the  oil,  and  is  then  mixed  thoroughly 
by  harrowing  again  before  being  compacted  by  the 
roller.  The  method  of  application  varies  with  the 
character  of  the  soil,  hard  soil  needing  to  be  loosened 
and  harrowed,  or  to  have  a  coating  of  sand  added, 
while  sandy  soils  may  be  oiled  without  being  disturbed. 

In  oiling  a  road  for  the  first  time,  two  applications 
of  oil  are  usually  made,  the  second  application  being 
made  at  from  one  week  to  three  months  after  the  first 
one.  In  most  instances  the  second  application  of  oil 
is  accompanied  by  a  thin  coating  of  sand  or  fine 
gravel,  which  takes  up  the  surplus  oil  and  forms  the 
wearing  surface  of  the  road. 

Oil.  The  oil  used  for  road  improvement  is  commonly 
crude  asphaltic  petroleum,  with  specific  gravity  1 1 
to  14  degrees,  Baume,  and  containing  from  30  to  60 
per  cent  of  "D"  grade  asphalt.  In  the  earlier  work 
the  oil  was  always  applied  hot,  at  temperatures  from 
150  to  250  degrees  Fahr.  In  later  practice  cold  oil 
has  frequently  been  used  and  each  method  has  its 
advocates. 


IMPROVEMENT   OF   COUNTE 

The  amount  of  oil  required  varies  with  the  charac- 
ter of  the  soil  and  the  method  of  treatment.  As 
much  should  be  used  as  the  soil  will  take  up.  The 
proper  amount  can  only  be  judged  by  experience  with 
the  soil  to  be  treated.  The  quantity  of  oil  used  varies 
from  about  one-half  gallon  to  ij  gallons  per  square 
yard  of  road  surface  for  the  first  application,  and 
one-half  to  I  gallon  for  the  second  application.  The 
maintenance  of  the  roads  usually  requires  an  applica- 
tion of  oil  each  spring,  the  quantity  required  decreas- 
ing from  year  to  year. 

PETROLITHIC    PAVEMENT. 

The  ordinary  oiled  road  consists  of  a  light  covering 
of  oiled  soil  upon  the  road  surface.  In  some  instances 
oiled  surfaces  are  made  4  to  6  inches  deep,  by  loosen- 
ing the  soil,  saturating  with  oil,  and  then  compacting 
by  rolling  to  a  firm  surface.  Difficulty  has  t>een  found 
in  compacting  so  deep  a  layer  of  oiled  earth,  and  this 
has  led  to  the  invention  of  the  petrolithic  rolling 
tamper  for  this  purpose.  The  rolling  tamper  is  shown 
in  Fig.  20.  It  consists  of  a  heavy  roller  with  a  large 
number  of  tampers,  or  feet,  projecting  from  its  surface. 
These  feet  pack  the  material,  beginning  at  the  bottom, 
and  thus  compress  the  whole  layer  to  uniform  density. 
An  oiled  road  constructed  with  this  machine  is  called 
a  petrolithic  pavement.  The  following  specifications 
used  in  Los  Angeles  for  this  class  of  streets  illustrate 
the  method  of  construction  employed  in  such  work: 

"After  the  street  has  been  brought  to  the  required 
grade  and  cross-section  as  above  specified,  the  sur- 
face shall  be  rolled  with  a  roller  weighing  not  less  than 
250  pounds  to  the  inch  width  of  tire  until  the  surface 


,  .A \rCEXT-SGfok  "ON  ROADS  AND  PAVEMENTS 

is  unyielding.     Depressions  made  by  the  rolling  shall 
be  leveled  up  with  good  earth  and  again  rolled.     Such 


FIG.  20. 

portions  of  the  street  as  cannot  be  reached  by  the 
roller,  and  all  places  excavated  below  grade  and 
refilled,  and  all  pipe  trenches  and  other  places  that 
cannot  be  properly  compacted  by  the  roller,  shall  be 
tamped  solid,  and  in  case  of  wet  weather  or  soft  or 
muddy  ground,  making  use  of  the  roller  unsafe  or 
impracticable,  the  rolling  shall  not  be  undertaken  until 
the  ground  has  become  sufficiently  dry. 

"  The  street  shall  then  be  tested  for  grade  and  cross- 
section,  and  no  further  work  shall  be  done  upon  it 
until  a  certificate  shall  have  been  issued  stating  that 
it  is  acceptable  in  these  respects.  It  shall  then  be 
plowed  to  a  depth  of  not  less  than  six  inches  and 
thoroughly  pulverized  by  cultivating  and  harrowing. 

OILING. 

"Oil  shall  then  be  applied  as  follows: 

"  The  area  to  be  oiled  shall  extend  from  curb  to  curb 


IMPROVEMENT   OF  COUNTRY   ROADS.  1 03 

where  there  are  no  gutters,  and  where  there  are  gutters 
then  from  gutter  to  gutter,  including  all  intersections 
of  streets  and  alleys,  and  to  the  property  line  on  both 
sides  of  said  intersections. 

"The  roadway  shall  be  coated  evenly  with  the  oil 
at  the  rate  of  one  gallon  to  the  square  yard  of  surface 
covered.  It  shall  then  be  thorough!}7  cultivated  to  a 
depth  of  4  inches  until  the  oil  is  well  mixed  with 
the  soil.  A  second  application  of  oil,  at  the  rate  of 
one  gallon  to  the  square  yard  of  surface  covered,  shall 
then  be  made  and  the  area  shall  be  again  cultivated 
to  a  depth  of  4  inches  until  the  oil  and  soil  are  well 
mixed.  The  street  shall  then  be  plowed  4  inches  deep 
with  a  plow  that  thoroughly  turns  over  the  furrows. 

"  A  third  application  of  oil,  to  the  extent  of  one  gallon 
per  square  yard  of  surface  covered,  shall  then  be  made, 
and  the  entire  surface  shall  be  thoroughly  cultivated 
to  a  depth  of  6  inches,  a  portion  of  the  cultivating 
being  done  along  diagonal  lines  so  as  to  thoroughly 
mix  the  surface.  The  road-bed  shall  then  be  tamped 
with  the  tamping  roller  until  it  is  solid  to  within  3 
inches  of  the  finished  surface.  It  shall  then  be  graded 
with  a  road  grader  until  it  substantial!}7  conforms  to 
the  official  cross-section,  and  shall  then  be  tamped  with 
the  tamping  roller  until  the  entire  surface  is  uniformly 
hard,  solid  and  free  from  undulations  or  other  irregu- 
larities. 

"  The  completed  surface  of  the  street  must  conform 
substantially  to  the  established  grade  and  cross- 
section.  Should  it  be  low,  it  shall  be  broken  up  to 
a  depth  of  at  least  2  inches,  fresh  earth  and  .oil  supplied 
and  the  surface  again  rolled  as  before. 

"  Should  an  excess  of  oil  remain  upon  the  surface 
after  it  has  been  thoroughly  completed,  such  oily 


IO4       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

portion  shall  be  plowed  to  a  depth  of  at  least  6  inches 
and  retamped. 

"  In  the  process  of  cultivating,  the  surface  shall  be 
gone  over  not  less  than  twice  after  each  of  the  first  two 
applications  of  oil,  and  not  less  than  three  times  after 
the  third  application,  and  in  all  cases  until  the  oil  and 
soil  are  thoroughly  mixed. 

"  The  total  quantity  of  oil  to  be  applied  on  the  street 
shall  be  not  less  than  three  (3)  gallons  net  oil  by 
measure  for  every  square  yard  of  surface  covered. 

"At  all  stages  of  the  work  sufficient  water  shall  be 
applied  to  secure  the  best  results  in  the  tamping,  the 
amount  of  water  to  be  used  to  be  governed  by  the 
character  of  the  soil,  the  intention  being  to  make 
the  soil  just  damp  enough  to  pack  solid. 

"Any  portion  of  the  street  that  cannot  be  reached 
by  the  roller  shall  be  tamped  solid  b}^  hand,  under  the 
direction  of  the  Board  of  Public  Works. 

"  The  contractor  will  be  held  responsible  for  all 
damage  to  curbs,  gutters,  or  cross-walks  that  may  be 
caused  by  him  in  the  performance  of  the  work. 


OIL. 

"The  oil  used  shall  be  crude"  petroleum  and  shall 
conform  to  the  following  requirements: 

"  (a)  Specific  Gravity :  The  oil,  after  being  freed  from 
water  and  sediment,  shall  be  of  not  less  than  eleven 
and  five-tenths  (11.5)  degrees,  and  not  more  than 
fourteen  (14)  degrees,  Baume,  gravity,  at  sixty  (60) 
degrees  F. 

"  The  specific  gravity  shall  be  determined  by  the  use 
of  'The  Westphal  Specific  Gravity  Balance/  in  con- 
junction with  the  accepted  scale  hereinafter  described 


IMPROVEMENT  OF  COUNTRY  ROADS      I 05 

for  addition  and  deduction  below  or  above  normal 
temperature. 

"(b)  Temperature:  All  oil  must  be  delivered  at  the 
point  required  for  sprinkling  at  a  temperature  of  not 
less  than  one  hundred  (100)  degrees  nor  more  than  one 
hundred  and  ninety  (190)  degrees  F. 

"(c)  Measurement:  In  determining  the  quantity  of 
oil  delivered,  the  correction  for  expansion  by  heat  shall 
be  as  follows :  From  the  measured  volume  of  all  oil 
received  at  any  temperature  above  60°  F.,  an  amount 
equivalent  to  0.4  of  one  per  cent  for  every  10°  F.  shall 
be  subtracted  as  the  correction  for  expansion  by  heat. 
For  the  purpose  of  measuring  oil  a  temperature  of 
60°  F.  shall  be  deemed  normal  temperature. 

"(d)  Volatility:  The  oil  shall  not  contain  more  than 
eight  (8)  per  cent  of  matter  volatile  when  said  oil  is 
heated  slowly  to  two  hundred  and  twenty  (220)  degrees 
F.  and  maintained  at  that  temperature  during  fifteen 
(15)  minutes. 

"(e)  Asphalt:  The  oil  shall  contain  not  less  than 
sixty  (60)  per  cent  of  asphalt,  having  at  a  temperature 
of  seventy-seven  (77)  degrees  F.  a  penetration  of  eighty 
(80)  degrees,  District  of  Columbia  Standard. 

"  The  percentage  of  asphalt  shall  be  determined,  using 
oil  treated  as  described  in  Section  (d)  in  the  following 
manner:  A  weighed  amount  of  said  treated  oil  shall  be 
heated,  in  an  evaporating  oven,  to  a  temperature  of 
four  hundred  (400)  degrees  F.  until  it  has  reached  the 
proper  consistency,  when  the  weight  of  the  residue 
shall  be  determined  and  the  per  cent  calculated. 

"  (f)  Water  and  Sediment:  Deduction  will  be  made 
for  water  and  sediment  in  exact  proportion  to  the 
percentage  of  such  water  and  sediment  found  therein, 
and  the  oil  shall  not  contain  over  two  (2)  per  cent  of 


106       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

such  water  and  sediment  as  determined  by  the  gasoline 
test. 

"  (g)  Tank  Wagons :  All  tank  wagons  used  for  deliver- 
ing the  oil  must  first  be  submitted  to  the  Department 
of  Oil  Inspection,  which  will  gauge  and  stamp  into  the 
steel  heads  of  said  tanks  the  capacity  in  gallons  of  said 
tanks,  and  no  figures  of  capacity  will  be  accepted  other 
than  the  official  rating  given  by  the  Department  of  Oil 
Inspection. 

"(h)  All  oil  to  be  used  shall  be  tested  by  the  Depart- 
ment of  Oil  Inspection. 

ROLLER. 

"The  tamping  roller  to  be  used  in  the  execution  of 
the  work  herein  specified  shall  consist  of  a  roller  the 
outer  surface  of  which  shall  be  studded  with  teeth  not 
less  than  7  inches  long  and  having  a  surface  area  of  not 
less  than  4  square  inches  each,  the  roller  itself  to  be  of 
such  a  weight  that  the  load  upon  each  tooth  shall  be 
not  less  than  300  pounds/' 

Results  of  the  Use  oj  Oil  on  Roads.  As  already  stated, 
good  results  seem  to  have  been  obtained  in  California 
with  the  use  of  oil  both  for  laying  dust  on  roads  and 
for  improving  the  resistance  to  wear  and  to  the  pene- 
tration of  water  into  the  road  surface.  The  results 
obtained  depend  upon  the  character  of  the  work  and 
the  care  used  in  construction.  To  secure  good  results 
these  roads  require  careful  maintenance.  The  forma- 
tion of  chuck  holes  due  to  the  action  of  water  upon  the 
road  is  a  principal  difficulty,  and  these  require  prompt 
repair.  A  lightly  oiled  road  surface  is  worn  away  by 
travel  and  water  during  the  rainy  season,  and  must 
be  annually  renewed.  On  the  whole  the  results  are 


IMPROVEMENT   OF   COUNTRY   ROADS.  IO/ 

reported  as  satisfactory  and  the  use  of  oil  is  largely 
extending. 

California  has  a  dry  climate,  which  is  very  favorable 
to  this  kind  of  construction.  The  object  of  the  road 
improvement  is  rather  to  get  rid  of  the  dust,  and  cause 
the  surface  of  the  roads  to  hold  together  during  the 
dry  season,  than  to  guard  against  the  softening  of  the 
roads  in  wet  weather.  Under  these  conditions  the  use 
of  oil  constitutes  a  very  desirable  method  of  road 
improvement;  while  the  occurrence  of  the  asphaltic 
oil,  which  ma}T  be  obtained  at  low  cost,  makes  possible 
economical  construction. 

In  considering  the  advisability  of  extending  such 
methods  to  other  parts  of  the  country,  the  differences 
of  climate  and  of  the  purpose  of  road  improvement 
should  be  taken  into  account,  as  well  as  the  character 
of  available  materials.  Some  method  of  dealing  with 
dust,  other  than  that  of  sprinkling  with  water,  is 
annually  growing  more  important,  while  the  breaking 
up,  or  raveling,  of  road  surfaces  during  dr}T  weather 
is  a  serious  difficulty,  particularly  where  there  is  con- 
siderable automobile  travel.  The  paraffine  oils  of  the 
Eastern  states  may  act  quite  differently  from  the 
California  asphaltic  oils,  while  the  greater  amount  of 
rainfall  and  differences  in  temperature  will  probably 
make  oiled  construction  much  more  difficult  to 
maintain  in  other  parts  of  the  country  than  in 
California. 

For  California  the  value  of  these  materials  has  been 
fully  demonstrated,  although  experience  is  likely  to 
modify  the  methods  of  construction  used.  For  other 
localities,  the  value  both  of  materials  and  methods 
can  be  determined  only  by  experiment. 


IO8       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  30.     SAND-CLAY  ROADS. 

In  some  of  the  Southern  states  where  other  materials 
for  surfacing  roads  are  not  available,  a  mixture  of  sand 
and  clay  in  proper  proportions  has  been  found  a  desir- 
able material  for  this  purpose.  In  some  instances 
the  mixing  of  sand  and  clay  for  road  work  has  received 
considerable  attention,  and  good  results  have  been 
obtained.  The  relative  amounts  of  sand  and  clay  to 
be  used  depend  upon  the  character  of  the  materials 
and  can  only  be  determined  by  experiment  in  each  case. 
Where  good  materials  are  available,  a  fairly  hard 
surface,  well  adapted  to  light  traffic,  may  be  obtained. 
It  is  claimed  that  these  roads  are  less  noisy  and  less 
dusty  than  macadam.  They  require  the  same  main- 
tenance as  an  ordinary  earth  road,  but  form  a  harder 
surface  than  earth  usually  found  in  a  natural  state. 
Mr.  William  L.  Spoon,  of  the  United  States  Office  of 
Public  Roads,  has  made  an  investigation  and  report 
upon  this  method  of  construction. 

*  "  The  best  sand-clay  road  is  one  in  which  the  wear- 
ing surface  is  composed  of  grains  of  sand  in  contact  in 
such  a  way  that  the  voids  or  angular  spaces  between 
the  grains  are  entirely  filled  with  clay,  which  acts  as  a 
binder.     Any  excess  of  clay  above  the  amount  necessary 
to  fill  the  voids  in  the  sand  is  detrimental.     If  a  small 
section  taken  from  the  surface  of  any  well  constructed 
sand-clay  road  is  examined  with  a  magnifying  glass, 
the   condition   of    contact   which    exists    between    the 
grains  of  sand  and  the  small  proportion  of  cla}7  which 
is  required  to  fill  the  voids  may  be  seen.     Wherever 
this  proper  condition  of  contact  exists  for  a  few  inches 

*  U.  S.  Department  of  Agriculture,  Office  of  Public  Roads,  Bulletin 
No.  27. 


IMPROVEMENT   OF   COUNTRY  ROADS.  1 09 

in  thickness  upon  the  surface  of  the  road,  it  will  bear 
comparatively  heavy  traffic  for  a  long  time,  even  when 
the  subsoil  is  sand  or  clay. 

"  All  the  experiments  which  have  been  made  by  this 
Office  indicate  that  the  materials  should  not  be  mixed 
in  a  dry  state,  but  that  they  should  be  thoroughly 
mixed  and  puddled  with  water.  It  makes  little 
difference  by  what  method  the  stirring  or  mixing  is 
done,  so  long  as  it  is  thorough  and  proper  proportions 
of  the  materials  are  obtained.  If  an  excess  of  clay  is 
used  in  the  mixture,  the  grains  of  sand  which  are  not  in 
contact  are  free  to  move  among  and  upon  each  other,  so 
that  no  particle  exerts  more  resistance  to  pressure  than 
if  the  entire  mass  consisted  of  clay  alone.  On  the  other 
hand,  if  an  insufficient  amount  of  clay  is  used,  the  mix- 
ture will  lack  binding  power  and  will  soon  disintegrate. 

s<  It  has  been  pointed  out  that  thorough  stirring  and 
puddling  are  absolutely  essential  to  successful  sand- 
clay  construction.  This  is  most  easily  brought  about 
immediately  after  a  hard  or  prolonged  rain,  the  clay 
having  been  previously  spread  and  the  large  lumps 
broken  up  as  completely  as  possible.  The  surface 
should  then  be  covered  with  a  few  inches  of  sand  and 
plowed  and  harrowed  thoroughly  by  means  of  a  turning 
plow  and  a  cutawaj7  or  disk  harrow.  This  stage  of 
the  work  will  of  course  be  found  somewhat  disagree- 
able, leading,  as  it  does,  to  the  formation  of  a  thick, 
past}^  mud;  but  it  is  the  only  practicable  \vay  in  which 
the  necessary  mixing  can  be  accomplished.  Many 
experiments  have  been  tried  with  dry  mixing  of  the 
clay  and  sand,  but  all  have  been  more  or  less  unsuccess- 
ful. In  cases  where  the  plowing  and  harrowing  are 
considered  too  expensive,  the  mixing  may  be  left  to 
traffic.  This,  however,  inevitably  leads  to  a  muddy 


IIO       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS, 

road  surface  for  a  long  time,  although  finally  it  is 
possible,  by  a  proper  distribution  of  the  sand  upon  the 
clay,  to  bring  about  a  fairly  good  result,  even  by  this 
simple  method/' 

"  It  has  already  been  shown  that  the  best  mixture  for 
sand-clay  construction  is  one  in  which  there  is  just 
enough  clay  to  fill  the  void  in  the  sand,  thus  producing 
the  proper  cementing  bond  in  the  road  surface.  No 
exact  rules  can  be  laid  down  for  calculating  in  advance 
the  best  mixture.  It  must  be  remembered  that  the 
relation  of  weight  and  volume  will  vary  widely  in 
different  clays,  according  to  the  amount  of  water  which 
they  contain.  Some  clays,  especially  the  more  plastic 
varieties,  even  after  they  are  as  thoroughly  dried  as 
they  can  be  by  the  hottest  summer  sun,  will  still  hold 
as  much  as  20  per  cent  of  water.  This  water  is  known 
to  chemists  as  'water  of  combination/  because  it 
seems  to  be  either  combined  with  or  held  in  the  structure 
of  the  clay  particles  in  such  a  way  that  it  can  only  be 
driven  out  at  a  high  temperature.  It  is  apparent  from 
this  that  in  handling  a  clay  of  this  kind,  even  when  it 
seems  quite  dry,  each  ton  will  contain  400  pounds  of 
water  which  does  not  enter  into  the  consideration  of 
volume.  The  amount  of  clay  necessary  to  fill  the  voids 
in  any  given  sand  will  therefore  be  found  to  vary/' 

"Practical  experience  has  shown  that  the  tendency 
is  to  calculate  too  little  rather  than  too  much  sand  for 
given  amounts  of  clay,  and  almost  invariably  a  second 
and  even  a  third  application  of  sand  is  necessary  over 
and  above  the  calculated  amount.  It  often  happens 
that  clay  will  work  up  to  the  surface  under  the  action  of 
traffic,  in  which  case  an  extra  top  dressing  of  sand 
should  be  added  when  required.  " 

Upon  a  clay  subsoil  "the  foundation  having  been 


IMPROVEMENT  OF  COUNTRY  ROADS.      Ill 

properly  prepared,  the  surface  should  be  plowed  and 
harrowed  to  a  depth  of  about  4  inches  until  it  is  pul- 
verized as  completely  as  possible.  It  is  then  covered 
with  6  to  8  inches  of  clean  angular  sand.  The  sand 
should  be  spread  so  that  the  layer  is  thickest  at  the 
center  of  the  road,  following  in  general  the  same  method 
as  was  outlined  for  spreading  clay  upon  a  sandy  founda- 
tion. The  first  mixing  by  plow  and  harrow  is  now 
done  while  the  materials  are  still  in  a  comparative^ 
dry  state.  It  has  been  found  that  the  clay  founda- 
tion can  be  more  evenly  disintegrated  when  in  that 
condition.  After  this  first  mixing  has  been  finished 
the  road  is  finally  puddled  with  a  harrow  after  a  rain. 
In  case  an  excess  of  clay  works  to  the  surface  and  tends 
to  make  the  mixture  sticky,  sand  is  applied  until  this 
trouble  is  overcome. 

"  Upon  the  completion  of  the  mixing  and  puddling, 
the  road  should  be  shaped  while  it  is  still  soft  enough 
to  be  properly  finished  with  a  scraper  and  at  the  same 
time  stiff  enough  to  pack  well  under  the  roller  or  under 
the  action  of  traffic.  In  case  it  is  impossible  to  obtain 
the  proper  consistency  of  the  surface  material,  it  is 
better  to  shape  the  road  when  somewhat  too  wet  than 
when  it  is  too  dry,  even  if  it  is  necessary  to  stop  traffic 
upon  it  for  a  few  days.  The  road  should  be  opened  to 
traffic  as  soon  as  practicable  after  completion,  as  this 
will  be  found  to  have  a  beneficial  effect  upon  it." 

ART.  31.     MISCELLANEOUS  ROADS. 

Corduroy  Roads.  In  timbered  country,  where  roads 
must  pass  over  wet  and  muddy  places,  corduroy  roads 
are  sometimes  employed.  They  are  built  by  laying 
logs  side  by  side  across  the  roadway.  By  taking 


112       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

sufficient  care  in  construction  to  select  the  logs  and 
even  up  the  spaces  between  them  with  smaller  pieces, 
a  reasonably  smooth  road  may  be  built.  Roads  of 
this  kind  are  at  best  very  rough  affairs,  and  require  a 
great  deal  of  work  to  keep  them  in  fair  condition  for 
travel.  They  are  of  value  only  as  temporary  roads 
across  bad  places  where  the  cost  of  a  better  road  would 
be  too  great. 

Plank  Roads.  Many  of  the  old  toll  roads  in  some 
parts  of  the  country  were  plank  roads,  and  at  one  time 
they  were  quite  common.  They  are  now  rarely  con- 
structed. The  road  is  built  of  planks  2  to  4  inches 
thick  and  8  or  9  feet  long,  laid  upon  two  rows  of  stringers 
about  5  feet  apart.  The  ends  of  the  planks  are  not  in 
line  but  are  stepped  off  to  assist  wagons  in  passing  from 
the  side  upon  them.  Usually  a  single  line  was  used  and 
teams  turned  out  upon  the  earth  road  at  the  side  in 
passing;  but  sometimes  a  double  track  was  provided 
for  teams  in  each  direction.  When  in  good  condition, 
roads  of  this  kind  may  be  very  good  for  travel,  but  they 
very  quickly  get  out  of  repair  and  are  not  economical. 

Shell  roads.  In  some  localities  where  oyster  shells 
are  plentiful,  these  are  used  in  constructing  roads. 
They  make  a  road  very  similar  to  that  built  of  a  soft 
limestone.  The  road  is  constructed  in  the  same 
manner  as  with  gravel,  the  shells  being  readily  com- 
pacted by  the  traffic;  and  binding  well  in  the  road.  The 
material  is  too  soft  to  resist  the  wear  of  heavy  traffic, 
and  grinds  up  rapidly  under  travel.  For  localities 
where  traffic  is  not  heavy  and  a  harder  road  covering 
would  be  expensive,  these  roads  have  often  been  found 
satisfactory  and  economical. 

Burnt-Clay  Roads.  In  certain  districts  in  the 
Southern  states,  sedimentary  clays  very  commonly 


IMPROVEMENT   OF   COUNTRY  ROADS.  1 13 

occur,  and  other  road  materials  are  not  available.  It 
has  been  proposed  to  form  a  road  surface  by  burning 
the  clay,  and  experiments  have  been  made  by  the 
United  States  Office  of  Public  Roads  which  seem  to 
indicate  that  in  many  instances  this  may  prove  an 
effective  means  of  road  improvement  in  such  localities. 

*  "  After  grading  the  road  to  an  even  width  between 
ditches,  it  is  plowed  up  as  deeply  as  practicable.  It 
\vill  be  found  necessary  to  use  four  horses  or  mules,  as 
the  extremely  hea\^  nature  of  the  clay  makes  the  work 
of  deep  plowing  difficult.  After  the  plowing  has  been 
completed,  furrows  are  dug  across  the  road  from  ditch 
to  ditch,  extending  through  and  beyond  the  width  to 
be  burned.  If  it  is  intended  to  burn  12  feet  of  roadway, 
the  transverse  furrows  should  be  1 6  feet  long,  so  as  to 
extend  2  feet  on  each  side  beyond  the  width  of  the  final 
roadway.  Across  the  ridges  formed  by  these  furrows  — 
which  should  be  about  4  feet  apart  —  the  first  course 
of  cord  wood  is  laid  longitudinally  so  as  to  form  a 
series  of  flues  in  which  the  firing  is  started.  From  15 
to  20  of  these  flues  are  fired  at  one  time. 

"The  best  and  soundest  cord  wood  is  selected  for 
this  course  and  should  be  laid  so  that  the  pieces  will 
touch,  thus  forming  a  floor.  Another  layer  of  wood  is 
thrown  irregularly  across  this  floor,  in  crib  formation, 
with  spaces  left  between  in  which  the  lumps  of  clay 
are  piled.  Care  should  be  taken  that  the  clay  placed 
on  this  cribbed  floor  is  in  lumps  coarse  enough  to  allow 
a  draft  for  easy  combustion. 

"  After  the  lumps  of  clay  have  been  heaped  upon  this 
floor,  another  course  of  wood  is  laid  parallel  to  the 
first.  The  third  layer  is  laid  in  exactly  the  same  manner ' 

*  U.  S.  Department  of  Agriculture,  Office  of  Public  Roads,  Bulletin 
No.  27. 


114       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

as  the  first,  and  each  opening  and  crack  should  be 
filled  with  brush,  chips,  bark,  small  sticks,  or  any  other 
combustible  material.  The  top  layer  of  clay  is  placed 
over  all  and  the  finer  portions  of  the  material  are  heaped 
over  the  whole  structure.  A  careful  arrangement  of 
this  cord  wood  cribbing  to  separate  the  clay  is  impor- 
tant, and  the  directions  should  be  carefully  followed. 

"  The  deep  covering  of  clay  which  is  thrown  over  all 
should  be  taken  from  the  side  ditches,  and  may  be  in 
lumps  of  all  sizes,  including  the  very  finest  material. 
It  is  spread  as  evenly  as  possible  over  the  top  in  a 
layer  of  not  less  than  6  to  8  inches.  Finally  the 
whole  is  tamped  and  rounded  off  so  that  the  heat  will 
be  held  within  the  flues  as  long  as  possible.  When 
coal  slack  is  available  the  two  top  layers  of  wood 
may  be  omitted  and  the  coal  slack  thoroughly  mixed 
with  the  clay. 

"  It  is  necessary  to  get  the  fires  in  the  flues  well 
under  way  before  the  first  ^er  of  wood  is  burned 
through.  The  first  action  of  the  fire  is  to  drive  out 
the  water  contained  in  the  clay  before  the  actual  burn- 
ing and  clinkering  can  begin.  In  burning  the  gumbo 
clays  a  great  advantage  is  gained  from  the  organic  and 
vegetable  matter  which  is  contained  in  the  clay,  as 
that  in  itself  aids  combustion. " 

"After  the  firing  is  completed  not  only  the  portion 
of  the  clay  which  forms  the  top  of  the  kiln  but  the 
ridges  between  the  flues  should  be  burned  thoroughly, 
so  as  to  form  a  covering  of  burnt  clay  10  to  12  inches 
in  depth,  which,  when  rolled  down  and  compacted, 
forms  a  road  surface  of  from  6  to  8  inches  in  thickness. 
If  properly  burned,  the  material  should  be  entirely 
changed  in  character,  and  when  it  is  wet  it  should  have 
no  tendency  to  form  mud. 


IMPROVEMENT  OF  COUNTRY  ROADS.  11$ 

"When  the  material  is  sufficiently  cooled  the  road- 
bed should  be  brought  to  a  high  crown  before  rolling, 
in  order  to  allow  for  the  compacting  of  the  material. 
This  can  best  be  done  with  a  road  grader.  After  this 
the  rolling  should  be  begun  and  continued  until  the 
road-bed  is  smooth  and  hard.  The  finished  crown 
should  have  a  slope  of  at  least  one-half  inch  to  the 
foot. " 

Slag  Roads.  Blast  furnace  slag  is  used  in  some 
localities  as  a  material  for  surfacing  roads.  In  some 
instances  also  slack  from  coal  mines  is  used  in  the  same 
way.  Where  these  materials  are  available,  they  may 
provide  a  cheap  method  of  improving  the  surfaces  of 
roads  of  light  traffic.  Usually  these  materials  are 
rapidly  reduced  to  powder  under  any  considerable 
traffic;  in  some  instances,  however,  slag  may  be  ob- 
tained which  is  hard  and  tough  and  forms  a  desirable 
road  metal. 

ART.  32.     WIDTH  OF  TIRES. 

The  effect  of  the  width  of  wheel  tires  upon  the 
resistance  to  traction  has  already  been  mentioned  in 
Art.  2.  For  ordinary  roads,  not  in  soft  condition, 
tractive  resistance  is  somewhat  less  for  wide  than  for 
narrow  tires.  This  difference,  while  not  usually  very 
great,  is  sufficient  to  be  quite  appreciable  in  the  work 
of  hauling  heavy  loads  upon  the  roads.  Narrow  tires 
have  a  much  more  destructive  effect  upon  a  road 
surface  than  wide  ones,  and  from  the  point  of  view 
of  road  maintenance,  wide  tires  are  very  desirable. 
The  concentration  of  a  heavy  load  upon  narrow  wheel 
tires  affords  very  little  surface  of  contact  between 
the  wheel  and  road,  and  causes  the  wheel  to  indent  the 
road  surface,  giving  a  powerful  cutting  action.  The 


Il6       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

same  load  on  a  tire  of  sufficient  width  would  tend  to 
compact  the  road,  acting  like  a  roller,  and  if  these 
wheels  are  so  placed  as  not  to  run  in  the  same  track, 
the  difference  would  be  still  more  marked.  There  is, 
however,  no  advantage  in  an  excessive  width  of  tire. 
When  this  exceeds  4  or  5  inches,  upon  a  properly 
crowned  road,  the  tire  will  be  only  partially  in  contact 
with  the  road  and  the  load  will  be  carried  on  one  edge 
of  the  tire,  which  will  indent  the  road  surface. 

The  general  introduction  of  wide  tires  upon  vehicles 
traveling  our  highways  would  greatly  simplify  the 
problem  of  road  maintenance,  particularly  upon  earth 
roads.  This  fact  is  generally  admitted  and  appre- 
ciated by  road  builders,  but  the  practical  difficulties 
met  in  attempting  to  change  the  prevailing  system 
of  narrow  tires  has  been  too  great,  and  the  agitation 
for  wide  tires  has  not  as  yet  produced  much  effect. 
Ma%ny  propositions  have  been  made  looking  toward 
the  regulation  of  the  width  of  tires  by  law.  This  has 
not  met  with  much  success.  In  some  states  the  laws 
provide  for  a  rebate  upon  road  taxes  to  persons  using 
wide  tires  upon  wagon  wheels  used  for  highway 
transportation. 

The  usual  width  of  tire  upon  ordinar}^  wagons  is 
i^  or  if  inches.  For  the  best  effect  upon  the  high- 
ways, these  should  be  increased  so  as  to  vary  from 
about  3  to  5  or  6  inches,  according  to  the  load  for 
\vhich  the  wagon  is  designed. 

The  wide  tire  is  at  a  disadvantage  on  a  distinctly 
bad  road,  and  efforts  to  secure  the  adoption  of  wider 
tires  can  hardly  meet  with  much  success  until  very 
great  improvement  has  taken  place  in  the  character 
of  the  country  roads.  Wider  tires  should  naturally 
follow  better  roads  and  assist  in  maintaining  them. 


CHAPTER  V. 
BROKEN-STONE  ROADS. 

ART.  33.     DEFINITION. 

BROKEN-STONE  roads  consist  essential^  of  a  mass  of 
angular  fragments  of  rock  deposited,  usually  in  layers, 
upon  the  road-bed  or  a  foundation  prepared  for  it,  and 
then  consolidated  to  a  smooth  and  uniform  surface  by 
means  of  a  roller  or  by  the  action  of  the  traffic  which 
passes  over  it. 

There  are  two  commonly  recognized  systems  of  con- 
structing broken-stone  roads,  differing  in  the  nature  of 
the  foundation  employed,  and  known  respectively  by 
the  names  of  the  men  who  first  introduced  them  into 
English  practice  as  Telford  roads  and  Macadam  roads, 

Each  of  these  systems  has  been  greatly  modified  in 
use  since  the  time  of  its  founder,  and  each  name  is  now 
used  to  cover  a  general  class  of  constructions  differing 
very  materially  within  itself  as  applied  in  the  practice 
of  different  engineers.  Each  of  the  systems  also  has 
its  earnest  advocates,  who  contend  for  its  exclusive  use, 
and  numerous  controversies  have  been  the  result,  at 
the  conclusion  of  which  each  party  is  "of  the  same 
opinion  still. "  The  view  taken  by  different  road- 
builders  in  this  matter,  it  may  be  remarked,  appears  to 
be  the  result  usually  of  the  local  necessities  of  the 
vicinities  in  which  they  work,  and  of  the  skill  with  which 
the  different  systems  have  been  applied  in  v.-or1:  v:%Vh 
has  come  under  their  observations.  In  road- building, 

117 


Il8       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

as  in  any  other  class  of  engineering  works,  no  rigid 
rules  can  be  laid  down  for  universal  application;  each 
road  must  be  designed  for  the  place  it  is  to  occupy  and 
the  work  it  is  to  do. 

In  some  parts  of  this  country  natural  gravel  is  sub- 
stituted for  broken  stone  in  the  construction  of  these 
roads,  the  methods  of  construction  being  the  same  as 
in  using  broken  stone. 

ART.  34.     MACADAM  ROADS. 

Macadam  roads  as  commonly  constructed  consist  of 
two  or  more  layers  of  broken  stone,  each  layer  being 
rolled  to  a  firm  bearing  before  placing  the  next.  The 
broken  stone  is  usually  placed  directly  upon  the  earth 
road-bed. 

In  constructing  a  macadamized  roadway,  the  road- 
bed is  first  brought  to  the  proper  grade  in  the  usual 
manner,  and  rolled  to  a  uniform  surface.  The  surface 
of  the  road-bed  is  either  flat  or  raised  at  the  middle  to 
the  same  section  as  is  to  be  given  the  finished  road- 
surface.  The  inclined  form  is  usually  employed,  and 
seems  preferable  on  account  of  affording  better  drain- 
age in  case  any  water  finds  its  way  through  the  surface 
layer. 

On  village  streets  where  curb  and  sidewalks  are 
employed,  this  section  of  the  road-bed  may  extend  to 
the  curbing  (as  shown  in  Fig.  3) ,  but  on  country  roads  a 
bench  of  earth  should  be  left  at  the  side  between  the 
broken  stone  and  the  gutter  in  order  to  confine  the 
broken  stone  while  it  is  being  compacted,  and  prevent 
the  spread  of  the  surface  materials.  The  form  of  the 
road-bed  before  placing  the  stone  would  then  be  as 
shown  in  Fig.  21,  where  the  completed  road  is  to  be  of 


BROKEN-STONE   ROADS.  119 

the  form  given  ih  Figs.  5  and  17.  Where  the  road-bed 
is  in  embankment,  it  is  common  to  construct  the  earth 
embankment  to  the  height  of  the  finished  surface,  and 
afterwards  excavate  the  material  necessary  to  admit  of 
placing  the  surface  la\Ters.  The  embankment  should 
be  allowed  to  settle  and  become  thoroughly  compacted 
before  the  broken  stone  is  placed  upon  it,  and  it  is 
desirable  with  new  embankments  that  they  be  used  for 
a  short  time  by  the  traffic  upon  the  earth  surface  be- 


FlG     21. 

fore  finishing  the  road;  where,  however,  the  material 
is  well  compacted  in  construction  and  can  be  thor- 
oughly rolled  this  is  not  necessary. 

In  constructing  the  road-bed  its  proper  drainage 
must  be  considered,  and  where  necessary  to  prevent  its 
becoming  wet  under  the  broken  stone  some  means 
should  be  adopted  to  artificially  drain  it. 

Upon  the  completion  of  the  road-bed,  a  \ayer  of 
broken  stone,  usually  from  3  to  5  inches  in  thickness,  is 
placed  upon  it  and  thoroughly  rolled.  Upon  this  a 
second  layer  is  placed  and  likewise  rolled  to  a  uniform 
surface.  Sometimes  a  third  layer  is  added,  or  in  case 
of  a  very  thin  road  it  may  consist  of  a  single  layer,  the 
number  of  la3Ters  depending  upon  the  thickness  of  the 
road.  When  no  roller  is  used,  the 'stone  is  usually 
spread  on  the  surface  of  the  road-bed  to  the  full  thick- 
ness desired  for  the  road,  and  left  to  the  action  of  the 
traffic. 

The  upper  layer  constitutes  the  wearing  surface  of 
the  road,  and  upon  this  it  is  usually  necessary  to  place 


I2O       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

a  thin  layer  of  finer  material  called  binding  material, 
which  may  consist  of  rock  chips,  sand,  small  gravel,  or 
sometimes  loam,  and  is  washed  and  rolled  into  the  inter- 
stices of  the  rock,  with  the  object  of  forming  a  com- 
pact and  impervious  surface.  Binding  material  is  in 
like  manner  often  added  to  the  lower  layers  of  the 
road,  although  this  has  not  been  common  practice. 
The  object  should  be  to  fill  the  voids  in  the  rock  as 
completely  as  possible,  serving  to  make  the  road  one 
solid  mass,  to  bind  the  rock  more  firmly  together,  and 
to  prevent  the  percolation  of  water  through  the  surface. 

ART.  35.     TELFORD  FOUNDATIONS. 

The  distinguishing  feature  of  a  telford  road  is  its 
paved  foundation.  It  consists  essentially  of  a  pave- 
ment of  stone  blocks  set  upon  the  road-bed  and  cov- 
ered with  one  or  more  layers  of  broken  stone. 

In  forming  a  telford  road  the  road-bed  is  con- 
structed in  the  same  manner  as  for  macadam,  being 
made  either  level  or  crowned.  A  pavement  is  then 
placed  upon  the  road-bed  from  5  to  8  inches  thick,  de- 
pending upon  the  thickness  to  be  given  the  road 
material,  the  general  practice  being  to  make  the  pave- 
ment about  two  thirds  of  the  total  thickness  of  the 
road.  The  stones  used  for  the  pavement  may  vary 
from  2  to  4  inches  in  thickness  and  8  to  12  inches  in 
length;  they  are  set  upon  their  widest  edges  and  with 
their  greatest  lengths  across  the  road.  The  irregulari- 
ties of  the  upper  part  of  the  pavement  are  then  broken 
off  with  a  hammer,  and  all  the  interstices  filled  with 
stone  chips  and  wedged  with  a  light  hammer  so  as 
to  form  a  completed  pavement  of  about  the  thickness 
required. 


BROKEN-STONE  ROADS.  121 

Upon  this  pavement  the  layers  of  broken  stone  are 
placed,  and  the  road-surface  completed  in  the  same 
manner  as  for  a  macadam  road. 

The  practice  of  Telford  was  to  grade  the  road-bed 
flat,  and  then  construct  his  pavement  deeper  in  the 
middle  than  at  the  sides,  using  for  a  roadway  1 6  feet 
wide  stones  about  8  inches  deep  at  the  middle  and 
5  inches  at  the  sides.  This  practice  is  still  followed  by 
some  engineers,  but  it  is  now  more  common  and  usually 
considered  preferable  to  make  the  surface  of  the  road- 
bed parallel  to  the  finished  surface  and  the  pavement 
of  uniform  thickness.  Fig.  22  shows  a  section  of  tel- 
ford  road  as  now  commonly  constructed. 


FIG.  22. 

The  following  extract  from  the  specifications  of  Mr. 
James  Owen,  for  telford  roads  in  Essex  County,  New 
Jersey,  may  be  regarded  as  representing  the  best  prac- 
tice in  such  construction. 

"After  the  road-bed  has  been  formed  and  rolled,  as 
above  specified,  and  has  passed  the  inspection  of  the 
Engineer  and  Supervisor,  a  bottom  course  of  stone, 
of  an  average  depth  of  —  -  inches,  is  to  be  set  by 
hand  as  a  close,  firm  pavement,  the  stones  to  be  placed 
on  their  broadest  edges  lengthwise  across  the  road  in 
such  manner  as  to  break  joints  as  much  as  possible, 
the  breadth  of  the  upper  edge  not  to  exceed  four  (4) 
inches.  The  interstices  are  then  to  be  filled  with 
stone  chips,  firmly  wedged  by  hand  with  a  hammer, 
and  projecting  points  broken  off.  No  stone  of  greater 


122       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

length  than  ten  (10)  inches  or  width  of  four  (4)  inches 
shall  be  used,  except  each  alternate  stone  on  outer 
edge,  which  shall  be  double  the  length  of  the  others 
and  well  tied  into  the  bed  of  the  road;  all  stones  with 
a  flat  smooth  surface  must  be  broken,  the  whole  sur- 
face of  this  pavement  to  be  subject  to  a  thorough 
settling  or  ramming  with  heavy  sledge  hammers,  and 
thoroughly  rolled  with  a  —  —  ton  -  -  roller.  No  stone 
larger  than  one  and  one-half  (i^)  inches  to  be  left 
loose  on  top  of  telford. ' 

The  proper  foundation  to  be  used  for  a  broken-stone 
road  depends  upon  the  nature  and  condition  of  the 
road-bed  upon  which  it  is  to  be  constructed  and  the 
nature  of  the  traffic  to  pass  over  it.  If  a  firm,  well- 
compacted,  and  thoroughly  drained  road-bed  may  be 
obtained,  of  material  which  will  not  readily  soften 
under  the  action  of  moisture,  there  will  usually  be  no 
need  for  a  special  foundation,  but  the  first  layer  of  the 
macadam  may  be  placed  directly  upon  the  surface  of 
the  road-bed.  If,  however,  the  road-bed  is  of  a  ma- 
terial retentive  of  moisture,  not  thoroughly  drained, 
and  likely  to  become  soft  in  wet  weather,  and  the 
broken  stone  be  laid  immediately  in  contact  with  it, 
the  stones  of  the  lower  layer  of  macadam  may  be  grad- 
ually worked  down  by  the  weight  of  the  traffic  into  the 
soft  earth,  and  the  soil  at  the  same  time  work  up  into 
the  voids  in  the  stone,  causing  a  gradual  disintegration 
of  the  road.  It  may  thus  also  become  retentive  of 
moisture  and  subject  to  the  disrupting  action  of  frost. 
In  this  case  some  foundation  must  be  provided  which 
is  capable  of  resisting  the  penetrating  action  of  the  soft 
material  of  the  road-bed  and  of  distributing  the  load 
over  it. 

It  is  not  intended  in  the   above  to   imply  that   the 


BROKEN-STONE   ROADS.  123 

use  of  a  foundation  of  this  character  should  take  the 
place  of  proper  drainage.  The  advisability  of  artificial 
drainage  should  always  be  carefully  considered,  and 
where  the  road  is  threatened  by  water  which  may  be 
removed  by  the  construction  of  drains  they  should  be 
used,  but  frequently  thorough  drainage  is  difficult  or 
doubtful,  and  it  is  desirable  to  adopt  heavy  construc- 
tion such  as  the  telford  foundation  gives. 

It  is  commonly  claimed  by  the  advocates  of  the 
macadam  system  of  construction  that  on  any  well- 
drained  and  well-compacted  road-bed  there  will  be  no 
tendency  on  the  part  of  the  stone  to  work  down  or  of 
the  soil  to  work  up,  and  hence  that  the  Telford  foun- 
dation is  an  unnecessary  expense.  The  difficulty  of 
procuring  a  perfectly  stable  and  reliable  road-bed  in 
many  localities  is,  however,  very  generally  recognized, 
and  telford  pavements  are  largely  used. 

The  Massachusetts  Highway  Commission  has  dis- 
continued the  use  of  telford  construction.  In  their 
report  for  1903  they  give  the  following  as  their  reasons 
for  this  course: 

"No  telford  foundations  have  been  laid  for  two 
years  past.  Much  of  this  class  of  construction  has 
been  done  by  the  commission,  and  every  contingency 
was  supposed  to  have  been  carefully  considered. 
Notwithstanding  the  careful  attention  to  details,  the 
results  from  the  use  of  telford  have  been  far  from 
satisfactory.  In  a  few  cases  the  large  stones  have 
come  to  the  surface  in  a  manner  which  would  seem 
to  indicate  a  movement  due  to  frost  action.  In  other 
cases,  where  a  fairly  soft  native  stone  was  used  for 
surfacing,  the  upper  courses  were  worn  away  so  as 
to  leave  the  large  stones  exposed.  There  are  few, 
if  any,  cases  where  equally  good  results  cannot  be 


124       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

obtained  by  the  use  of  sand,  gravel,  or  small  stones 
in  place  of  telford,  and  at  a  less  cost/' 

These  difficulties  do  not  seem  to  have  been  met  by 
other  road  builders,  and  where  conditions  are  such  as 
to  make  advisable  the  construction  of  thick,  heavy 
roads,  telford  construction  is  very  commonly  adopted 
as  the  cheapest  form  for  such  work. 

The  Massachusetts  Highway  Commission  has  also 
adopted  for  difficult  construction  on  wet,  heavy  soils, 
a  blind  center  drain,  and  consider  this  cheaper  than 
the  telford  construction.  This  is  described  in  their 
report  for  1904  as  follows: 

"On  heavy,  wet  soils  a  center  'V '-shaped  drain 
has  been  substituted  for  the  side  drains  and  telfording. 
In  building  this  type  of  road  the  earth  is  loosened  and 
thrown  out  toward  the  sides,  so  as  to  give  a  'V- 
shaped  trench,  with  its  greatest  depth  in  the  center 
of  the  proposed  roadway.  Narrow  trenches  are  cut 
through  the  sides  of  this  center  trench,  at  intervals 
of  50  or  more  feet,  connecting  its  lowest  part  with  the 
gutters  on  the  side,  and  placed  at  a  depth  and  slope  to 
thoroughly  remove  all  water.  The  center  and  cross 
ditches  are  filled  with  field  or  wall  stone,  the  depth 
of  this  stone  varying  from  12  to  1 8  inches  at  the 
center,  and  from  6  to  12  inches  on  the  sides,  the  thick- 
ness being  dependent  upon  the  character  of  the  soil 
in  the  sub-grade.  The  tops  of  these  large  stones  are 
given  a  crown  to  receive  the  surfacing  material/' 

ART.  36.     ROCKS  FOR  ROAD  BUILDING. 

Properties  Required.  The  surface  material  for  broken- 
stone  roads  must  bind  together  into  a  solid  surface 
capable  of  bearing  the  loads  which  come  upon  it  and 
of  resisting  the  wear  of  the  traffic. 


BROKEN-STONE  ROADS.  125 

A  stone  to  be  durable  in  the  surface  of  a  road  should 
be  as,  hard  and  tough  as  possible.  The  qualities  of 
toughness  and  resistance  to  abrasion  are  of  more 
importance  than  hardness  and  resistance  to  crushing. 
A  stone  may  be  hard  and  brittle  and  quickly  pound 
to  pieces  in  a  road  surface,  or  it  may  have  a  high 
crushing  strength  and  grind  away  quickly  under 
abrasion,  as  is  the  case  with  some  varieties  of  sand- 
stone. If,  however,  it  be  too  soft,  it  may  crush 
under  the  loads  coming  upon  it,  and  thus  lack  in 
durability. 

A  stone  for  a  road-surface  must  also  resist  well  the 
disintegrating  influences  of  the  atmosphere.  It  should 
be  as  little  absorptive  .of  moisture  as  possible  in  order 
that  it  may  not  be  liable  to  injury  from  the  action 
of  frost.  Many  limestones  are  objectionable  on  this 
account. 

The  material  of  a  road-surface  should  also  be  uni- 
form in  quality;  otherwise  the  wear  of  the  surface  will 
not  be  even,  and  depressions  will  appear  where  the 
softer  material  has  been  placed. 

As  the  under  parts  of  the  road  are  not  subject  to 
the  wear  of  the  traffic,  and  have  only  the  weight  of  the 
loads  to  sustain,  it  is  evidently  not  important  that  the 
foundation  or  lower  layers  be  of  so  hard  or  tough  a 
material  as  the  surface;  and  hence  it  is  frequently  pos- 
sible, by  using  an  inferior  stone  for  that  portion  of  the 
work,  to  greatly  reduce  the  cost  of  construction. 

The  binding  of  the  road-surface  into  a  compact  mass 
capable  of  resisting  the  wear  of  traffic  depends  largely 
upon  the  cementing  properties  of  the  material.  By 
the  cementing  power  of  the  stone  is  meant  that 
property  which  enables  the  fine  dust  to  act,  when  wet, 
as  a  cement  and  bind  the  fragments  of  rock  composing 


126       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  road-surface  firmly  together.  This  is  perhaps  the 
most  important  quality  of  the  material,  and  a- high 
cementing  value  is  always  desirable.  The  tenacity 
with  which  the  fragments  of  rock  are  held  together  is 
perhaps  more  important  in  the  wear  of  the  road  than 
the  resistance  to  wear  of  the  fragments  themselves. 
The  powdering  of  the  cementing  material  in  dry  weather 
sometimes  causes  the  loosening  of  the  stones  and  the 
raveling  of  the  broken-stone  surface.  This  is  more 
apt  to  occur  where  the  road  metal  is  hard  and  resistant 
to  wear  than  where  it1  grinds  up  more  rapidly. 

The  character  of  the  material  which  will  give  the 
best  results  in  a  road-surface  depends  upon  the  local 
conditions  under  which  the  road  is  to  be  built  and 
the  traffic  to  which  it  is  to  be  subjected.  Under  heavy 
traffic,  hard  and  tough  road  metal  is  necessary  to 
good  results.  Under  lighter  traffic,  a  softer  rock  may 
sometimes  be  better  if  it  is  coupled  with  good  binding 
properties. 

*  "Experience  shows  that  a  rock  possessing  all 
three  of  the  properties  mentioned  in  a  high  degree 
does  not  under  all  conditions  make  a  good  road  material; 
on  the  contrary  under  certain  conditions  it  may  be 
altogether  unsuitable.  As  an  illustration  of  this,  if 
a  country  road  or  city  parkway,  where  only  a  light 
traffic  prevails,  were  built  of  a  very  hard  and  tough 
rock  with  a  high  cementing  value,  neither  the  best 
nor,  if  a  softer  rock  were  available,  the  cheapest  results 
would  be  obtained.  Such  a  rock  would  so  effectively 
resist  the  wear  of  a  light  traffic  that  the  amount  of 
fine  dust  worn  off  would  be  carried  away  by  wind  and 
rain  faster  than  it  would  be  supplied  by  wear.  Conse- 
quently the  binder  supplied  by  wear  would  be  insuffi- 

*  Engineering  Record,  May  17,  1902. 


BROKEN-STONE   ROADS.  I2/ 

cient,  and  if  not  supplied  from  some  other  source  the 
road  would  soon  go  to  pieces.  The  first  cost  of  such 
a  rock  would  in  most  instances  be  greater  than  that  of 
a  softer  one,  and  the  necessary  repairs  resulting  from 
its  use  would  also  be  very  expensive." 

The  selection  of  material  for  road  metal  is  commonly 
determined  rather  by  the  cheapness  and  convenience 
of  location  than  by  its  desirability  for  the  purpose.  In 
most  instances  this  is  of  necessity  the  case,  and  the 
availability  of  material  in  vicinity  of  the  work  makes 
possible  the  construction  of  the  road.  It  is,  however, 
frequently  possible,  by  judicious  selection  of  materials, 
to  greatly  improve  the  results  obtained  in  such  work, 
and  while  the  selection  of  a  stone  for  road  construction 
wrill  of  course  always  depend  largely  upon  what  is  to 
be  obtained  in  the  locality  of  the  work,  the  importance 
of  a  thoroughly  good  material  in  the  road  surface  is 
so  great  in  its  effect  upon  the  durability  and  cost  of 
repairs  of  the  road  that  it  may  frequently  be  found 
economical,  on  roads  subjected  to  a  considerable  traffic, 
to  bring  a  good  material  a  considerable  distance  rather 
than  to  use  an  inferior  one  from  the  immediate  vicinity. 
It  may  also  be  suggested  in  this  connection  that  in 
many  instances  railway  transportation  over  a  consid- 
erable distance  may  be  small  compared  with  wagon 
transportation  over  a  short  distance,  and  the  impor- 
tation of  good  material  may  add  but  slightly  to  the 
aggregate  cost  of  the  work. 

Classification.  The  rocks  used  for  road-building 
differ  widely  in  their  mineral  characters.  The  classifi- 
cation shown  in  the  following  table  is  proposed  by 
Mr.  Edwin  C.  E.  Lord.* 

*  U.  S.  Department  of  Agriculture,  Office  of  Public  Roads, 
Bulletin  No.  31.  1907. 


128        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 


GENERAL    CLASSIFICATION    OF    ROCKS. 


.    Intrusive  (plutonic) 


{a.    Rhyolite. 
c.    Andesite. 


ld. 

Basalt  and  diabase. 

I1- 

Calcareous.  .  .  . 

..I3' 

IQ 

Limestone. 
Dolomite. 

fa.' 

Shale. 

u. 

Siliceous  

Jb 

ocinQstoric* 

I 

Chert  (flint). 

a. 

Gneiss. 

I. 

Foliated  

1 

Schist. 

'  '  tc.' 

Amphibolite. 

,a. 

Slate. 

2. 

Nonfoliated.  .  . 

••••    c' 

Eclogite. 

•m    m-            11 

I.   Igneous.. .... 


II.    Sedimentary. . . 


III.    Metamorphic. . . 


"  All  rocks  of  the  igneous  class  are  presumed  to  have 
solidified  from  a  molten  state,  either  upon  reaching  the 
earth's  surface  or  at  varying  depths  below  it.  The 
physical  conditions,  such  as  heat  and  pressure,  under 
which  the  molten  rock  magma  consolidated,  as  well  as 
its  chemical  composition  and  the  presence  of  included 
vapors,  are  the  chief  features  influencing  the  structure. 
Thus,  we  find  the  deep-seated,  plutonic  rocks  coarsely 
crystalline  with  mineral  constituents  well  defined,  as 
in  case  of  granite  rocks,  indicating  a  single,  prolonged 
period  of  development,  whereas  the  members  of  the 
extrusive,  or  volcanic,  types,  solidifying  more  rapidly 
at  the  surface,  are  either  fine  grained  or  frequently 
glassy  and  vesicular,  or  show  porphyritic  structure. 
This  structure  is  produced  by  the  development  of  large 
crystals  in  a  more  or  less  dense  and  fine-grained  ground 
mass,  and  is  caused  generally  by  a  recurrence  of  mineral 
growth  during  the  effusive  period  of  magnetic  consoli- 


BROKEN-STONE   ROADS.  I2Q 

dation.  Rocks  of  this  kind,  exhibiting  a  more  or  less 
spotted  appearance,  are  commonly  described  as  por- 
phyries, regardless  of  mineral  composition,  thus  causing 
great  confusion  in  the  nomenclature.  A  movement 
in  the  rock  magma  while  cooling  causes  frequently  a 
banded  arrangement  of  the  minerals,  or  flow  structure." 

"Igneous  rocks  vary  in  color  from  the  light  gray, 
pink,  and  brown  of  the  acid  granites,  syenites,  and 
their  volcanic  equivalents  (rhyolite,  andesite,  etc.)  to 
the  dark  steel  gray  or  black  of  the  basic  gabbro, 
peridotite,  diabase,  and  basalt.  The  darker  varieties 
are  commonly  called  trap.  This  term  is  in  very  general 
use  and  is  derived  from  trappa,  Swedish  for  stair, 
because  rocks  of  this  kind  on  cooling  frequently  break 
into  large  tabular  masses,  rising  one  above  the  other 
like  steps,  as  may  be  seen  in  the  exposures  of  diabase 
on  the  west  shore  of  the  Hudson  River  from  Jersey 
city  to  Haverstraw. 

"  The  sedimentary  rocks  as  a  class  represent  the  con- 
solidated products  of  former  rock  disintegration,  as  in 
case  of  sandstone,  conglomerate,  shale,  etc.,  or  they 
have  been  formed  from  an  accumulation  of  organic 
remains  chiefly  of  a  calcareous  nature,  as  is  true  of 
limestone  and  dolomite.  These  fragmental  or  clastic 
materials  have  been  transported  by  water  and  deposited 
mechanically  in  layers  on  sea  or  lake  bottoms,  producing 
a  very  characteristic  bedded  or  stratified  structure  in 
many  of  the  resulting  rocks." 

"  Metamorphic  rocks  are  such  as  have  been  produced 
by  the  prolonged  action  of  physical  and  chemical 
forces  (heat,  pressure,  moisture,  etc.)  on  both  sedi- 
mentary and  igneous  rocks  alike.  The  foliated  types 
(gneiss,  schist,  etc.)  represent  an  advanced  stage  of 
metamorphism  on  a  large  scale  (regional  metamor- 


130       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

phism),  and  the  peculiar  schistose  or  foliated  structure 
is  due  to  the  more  or  less  parallel  arrangement  of 
their  mineral  components.  The  nonfoliated  types 
(quartzite,  marble,  slate,  etc.)  have  resulted  from  the 
alteration  of  sedimentary  rocks  without  materially 
affecting  the  structure  and  chemical  composition  of 
the  original  material/' 

The  rocks  commonly  used  for  road-building  may  be 
classified  according  to  their  popular  designations  as 
trap,  granite,  limestone,  sandstone,  and  chert. 

Trap.  The  term  "trap"  is  commonly  applied  to  most 
of  the  volcanic  igneous  rocks  used  for  road-building, 
including  basalt  and  diabase.  While  these  rocks  vary 
considerably  in  character,  they  are  usually  very  com- 
pact and  tough,  and  may  be  classed  as  the  best  material 
for  roads  of  heavy  traffic. 

*  "  It  is  characteristic  of  all  the  trappean  rocks  that 
they  have  once  been  fluid  from  heat  and  while  in  that 
state  have  been  injected  into  fissures  of  the  rocks 
through  which  they  have  found  their  way  toward  the 
present  surface  of  the  country.  Only  in  rare  cases 
have  they  actually  passed  upward  to  the  surface  of 
the  earth  toward  which  they  moved;  their  motion  was 
arrested  in  the  lower  levels  of  the  rocks  to  which  the 
surface  has  been  brought  down  by  the  agents  of  atmos- 
pheric decay.  The  result  of  their  consolidation  under 
the  conditions  of  pressure  in  which  they  cooled  has 
caused  these  originally  molten  materials  to  be  very 
compact,  a  state  which  is  favored  also  by  their  chemical 
composition.  This  causes  the  materials  to  be  very 
solid  and  elastic.  They  generally  resist  decay  in  such  a 
manner  that  they  often  project  above  the  surface,  while 
the  softer  rocks  on  either  side  have  been  worn  down." 

*  Shaler's  American  Highways,  p.  56. 


BROKEN-STONE  ROADS.  131 

Granite.  The  granites,  including  syenite  and  gneiss, 
vary  widely  in  character  and  differ  greatly  in  value  as 
road  materials.  They  may  be  classed  as  next  in  value 
to  trap  for  wear  in  the  road-surface,  but  are  somewhat 
deficient  in  cementing  properties. 

*  "  In  an  examination  of  the  bearing  of  the  petro- 
logical  characters  upon  the  attrition  results  in  this 
group  three  prominent  factors  stand  out.  They  are: 
(i)  Texture,  (2)  the  kind  of  mineral,  (3)  the  state  of 
freshness  of  the  minerals.  With  regard  to  the  first  of 
these  it  is  evident  that  fineness  and  evenness  of  grain 
is  an  advantage,  and  that  coarse  grain  or  porphyritic 
structure  is  disadvantageous.  It  is  on  account  of  the 
granitic  texture  that  the  rocks  of  this  group,  taken  as 
a  whole,  are  not  higher  in  the  attrition  scale. 

"The  influence  of  the  kind  of  mineral  (2)  is  not  so 
easy  to  determine,  but,  other  things  being  equal,  a 
high  proportion  of  hornblende  appears  to  be  favorable 
to  resistance;  quartz  in  a  like  manner  is  favorable 
because  of  its  hardness  and  lack  of  cleavage. 

"  Fresh  unaltered  original  minerals  are  not  absolutely 
essential  to  a  high  capacity  to  resist  abrasion;  the  two 
stones  that  take  the  best  position  in  the  test  scale  for 
this  group  are  considerably  altered  —  the  feldspars  are 
decomposed,  and  their  substance  is  a  mixture  of  smaller 
mineral  units;  the  ferro-magnesian  minerals  have 
changed  to  chlorite  and  to  fibrous  uralitic  hornblende." 

|  "  In  the  case  of  the  igneous  rocks  it  will  be  noted 
that  the  plutonic  types  with  granitic  granular  structure 
(granite,  syenite,  diorite,  and  gabbro)  are,  as  a  rule, 
harder  but  inferior  in  toughness  to  their  volcanic 

*  Lovegrove,  Attrition  Tests  of  Road-making  Stones,  p.  59. 

t  U.  S.  Department  of  Agriculture,  Office  of  Public  Roads,  Bulletin 


132       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

equivalents  (rhyolite,  basalt,  and  diabase).  This  is 
due  to  the  more  fully  crystalline  condition  and  coarser 
grain  of  the  plutonic  rocks.  In  the  case  of  the  volcanic 
types  a  compact  crystal  intergrowth  and  fine  grain 
tend  to  increase  the  toughness  rather  than  hardness  of 
the  material.  The  deleterious  effect  of  atmospheric 
decomposition  on  rock  texture  is  especially  noticeable 
in  the  case  of  peridotite,  andesite,  and  altered  basalt, 
where  the  indifferent  results  of  the  physical  tests, 
excepting  cementing  value,  may  be  directly  ascribed 
to  the  presence  of  such  soft  secondary  minerals  as 
kaolin,  serpentine,  calcite,  chlorite,  etc.  ...  As  has 
already  been  stated  in  a  previous  paragraph,  the 
cementing  value  is  as  a  rule  found  more  highly 
developed  in  the  igneous  rocks  which  contain  alteration 
products  than  in  their  unaltered  varieties.  This  is 
especially  true  in  the  case  of  diabase  and  basalt,  rocks 
very  similar  in  origin  and  mineral  composition.  Con- 
tinuing a  step  further,  we  note  a  marked  decrease  in 
toughness,  hardness,  and  resistance  to  wear  in  the 
altered  varieties  of  both  these  rock  types  over  their 
fresher  representatives.  This  is  in  line  with  what  has 
already  been  said  and  indicates  that  the  presence  of 
secondary  minerals  in  appreciable  quantities,  whether 
because  of  their  softness  or  their  indefinite  semi- 
crystalline  condition,  weakens  the  original  mineral 
bond  and  tends  to  destroy  the  primary  texture  of  the 
rock,  while  at  the  same  time  furnishing  the  elements 
for  a  high  binding  quality  in  the  rock  powder.  Valuable 
results  bearing  on  the  decomposition  of  rock  powders 
by  water  have  been  obtained  by  Dr.  A.  S.  Cushman.in 
a  series  of  interesting  experiments  carried  on  in  the 
chemical  laboratory  of  this  Office.  Doctor  Cushman 
has  shown  that  hydrolysis  takes  place  in  case  of  many 


BROKEN-STONE   ROADS.  133 

rock  powders  the  moment  they  are  wet,  thus  pro- 
ducing secondary  products  (hydrated  silicates)  of  a 
colloidal  nature  which  greatly  increase  the  binding 
power.  This  points  finally  to  the  conclusion  that  the 
mineral  analysis  of  igneous  rocks,  besides  providing  a 
convenient  means  for  comparison  and  classification, 
serves  to  a  certain  extent  as  a  measure  of  their  physi- 
cal properties/' 

Limestone.  Limestones  commonly  possess  the 
cementing  power  in  fair  degree,  although  lacking  in 
hardness  and  resistance  to  abrasion.  The  cementing 
power  has  probably  been  commonly  overestimated, 
because  of  the  softness  of  the  rock  and  the  ease  with 
which  it  usually  packs  in  the  road  surface.  Limestones 
are  the  most  widely  distributed  and  most  generally 
used  materials  for  road  surfaces.  They  differ  very 
widely  in  character,  some  forming  an  excellent  material 
under  moderate  traffic  and  others  being  so  soft  as  to 
offer  little  resistance  to  wear. 

"In  proportion  as  limestone  becomes  crystalline, 
i.e.,  takes  on  the  character  of  marble,  its  value  in  road- 
making  diminishes,  for  the  reason  that  the  crystalline 
structure  in  most  cases  so  far  weakens  the  mass  that 
it  is  apt  readily  to  pass  into  the  state  of  powder.  As 
these  marbles  occur  only  in  districts  where  better  road- 
making  materials  are  likely  to  be  present,  they  may  not 
be  further  mentioned,  except  to  say  that  their  use  is 
commendable  for  foundation  layers,  where  their  fair 
cementation  value  makes  them  tolerably  fit  for  service. 
So  long  as  the  bits  are  kept  from  the  destructive  action 
of  the  wheels  and  feet  of  the  carriages  and  horses,  they 
lend  themselves  to  the  road-master's  use.  Even  where 
a  more  resisting  top  covering  of  ordinary  broken  stone 

*  Shaler*s  American  Highways,  p.  61. 


134       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

cannot  be  provided,  a  tolerable  road  can  be  made  of 
this  material,  often  very  cheaply  by  using  the  waste 
from  quarries,  by  covering  the  surface  with  a  coating 
of  ferruginous  matter,  such  as  is  afforded  by  the  leaner 
iron  ores,  or  by  using  a  top  coating  of  gravel." 

"  If  we  take  the  pure  dolomites  alone,  it  is  clear  that 
those  behave  best  in  the  attrition  test  which  have 
a  fine-grained,  even,  granular  texture  with  irregular- 
shaped  grains  interlocking  closely  one  with  another, 
and  with  a  general  absence  of  porous  cavities.  A  dolo- 
mite is  by  no  means  always  better  than  a  limestone, 
but  the  best  type  of  dolomite  will  be  more  resistant 
than  the  best  limestones,  being  harder. 

"  Among  the  limestones,  those  stand  highest  that  are 
composed  of  a  mixed  assemblage  of  small  organic 
remains,  notably  of  foraminifera,  and  possess  at  the 
same  time  a  somewhat  bituminous  composition  (this 
characteristic  is  often  associated  with  foraminifera  in 
carboniferous  limestone).  Crinoidal  limestones  do  not 
stand  so  high,  evidently  on  account  of  the  ready 
cleavage  of  the  particles  of  calcite,  which  is  not  only 
a  soft  mineral  but  has  an  extremely  perfect  cleavage, 
hence  it  wears  rapidly  and  crumbles  easily  under 
repeated  small  blows;  but  if  the  crinoid  fragments 
are  small  and  uniform  in  size,  set  in  a  matrix  of  fine 
calcareous  matter,  the  stone  may  compare  well  with 
other  limestones. " 

Sandstone.  As  a  class  sandstones  are  deficient  in 
cementing  power  and  do  not  stand  well  in  the  surface 
of  a  road.  They  have  commonly  been  sweepingly 
condemned  and  rejected  by  road-builders.  In  some 
instances,  however,  sandstones  have  given  good 
results,  and  some  of  them  possess  fairly  good  cementing 
*  Lovegrove,  Attrition  Tests  of  Road-making  Stones,  p.  61. 


BROKEN-STONE   ROADS.  135 

power.  The  value  of  a  sandstone  depends  mainly 
upon  the  character  of  the  cementing  medium;  where 
this  is  of  siliceous  character,  a  high  degree  of  hardness 
and  resistance  to  wear  may  result. 

"Considering  the  next  important  group  of  road- 
making  rocks,  we  notice  here  also  a  marked  coincidence 
in  mineral  composition  and  physical  properties.  The 
soft  and  nonresistant  calcareous  rocks  (limestones, 
dolomites, and  calcareous  sandstones),  composed  largely 
of  calcite  and  dolomite,  are,  as  would  be  expected, 
inferior  in  hardness,  toughness,  and  wearing  qualities 
to  the  more  siliceous  sandstones  and  cherts." 

Chert.  Chert  is  a  very  hard  material  and  shows 
good  resistance  to  wear.  It  is  somewhat  low  in  cement- 
ing value,  but  when  carefully  used  forms  a  good  road 
material.  It  is  quite  variable  in  character  and  needs 
careful  selection.  Chert  is  commonly  found  in  a 
finely  divided  condition,  and  can  be  used,  in  many 
instances,  without  crushing.  It  occurs  throughout 
many  of  the  Southern  states,  where  it  is  found  widely 
distributed  and  is  the  only  available  material  for  such 
work. 

*  "  The  low  cementing  value  of  chert  obtained  by 
laboratory  tests  is  not  in  every  case  in  accordance  with 
that    developed   by   this   rock   under  traffic.     In   dis- 
cussing the  origin  of  road  material  it  has  been  stated 
that  chert  or  flint  belongs  to  that  class  of  sedimentary 
rocks   whose   mineral   components  have   been   formed 
largely  by  chemical  precipitation  and  were  originally 
of    a    colloidal    or    amorphous    nature.     The    highly 
fractured  condition  of  many  cherts  is  probably  due  in 
large  measure  to  shrinkage  caused  by  a  decrease  in 

*  U.  S.  Department  of  Agriculture,  Office  of  Public  Roads,  Bulletin 


136       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

volume  in  passing  from  an  amorphous  to  a  crystalline 
state.  Although  no  experiments  have  as  yet  been  made 
on  the  solubility  of  this  material,  it  seems  to  the  writer 
very  probable  that  the  dissolving  action  of  road  waters 
on  finely  divided  chert  dust  is  relatively  high  and  that 
the  high  binding  power  of  some  of  these  rocks  is 
caused  by  hydrated  opaline  silica  resulting  from  a 
decomposition  of  this  kind.  The  fact  that  in  certain 
localities  surface  flints  are  superior  to  quarry  flints 
for  road  making  is  suggestive  in  this  connection/' 

Mr.  Lord,  in  the  bulletin  already  quoted,  gives  tables 
showing  the  average  mineral  composition  and  plates 
showing  structure  of  the  various  rocks,  and  indicates 
that  the  probable  value  of  a  rock  for  road-building 
may  be  inferred  from  its  mineral  composition  and 
structure. 

ft  To  explain  the  bearing  of  mineral  composition  and 
structure  on  the  physical  properties  of  rocks,  it  has 
been  found  necessary  to  define  these  properties  and 
describe  the  various  methods  for  testing  road  materials. 
The  results  of  these  tests  have  been  used  in  correlating 
the  physical  properties  of  the  various  rock  families 
with  their  mineral  components,  and  the  following 
conclusions  have  been  reached: 

"(i)  Igneous  and  metamorphic  rocks,  owing  to  a 
high  degree  of  crystallization  and  a  preponderance  of 
silicate  minerals,  offer  a  greater  resistance  to  abrasion 
than  nearly  all  varieties  of  sedimentary  rocks. 

"  (2)  The  coarse-grained  intrusive  rocks  of  the  igneous 
class  are  harder,  but  break  more  readily  under  impact 
than  the  finer-grained  volcanic  varieties  of  like  mineral 
composition. 

"  (3)  The  deleterious  effect  of  atmospheric  weathering 
on  the  wearing  qualities  of  rocks  has  been  demonstrated. 


BROKEN-STONE  ROADS.  137 

"(4)  The  cementing  value  of  rocks  is,  to  a  certain 
degree,  measured  by  the  abundance  of  secondary 
minerals  resulting  from  rock  decay. 

"  (5)  Metamorphic  rocks  have,  as  a  rule,  a  low  bind- 
ing power,  owing  to  a  regeneration  of  secondary 
minerals  and  to  the  effects  of  heat  and  pressure.  The 
foliated  types  part  readily  along  planes  of  schistosity 
and  therefore  are  not  well  adapted  to  road  con- 
struction. 

"(6)  The  quantitative  mineral  analysis  of  rocks 
serves  to  a  certain  extent  as  a  measure  of  their  useful 
properties  for  road  construction.  " 

ART.  37.     METHODS  OF  TESTING  STONE. 

Final  judgment  concerning  the  value  of  stone  for 
road  purposes,  or  the  best  method  of  using  it,  can  only 
be  formed  through  experience  with  the  material  in  use. 
Tests  may,  however,  be  applied  which  will  throw  much 
light  upon  the  probable  value  of  a  material,  or  which 
may  give  an  idea  of  the  probable  relative  values  of 
different  available  materials  in  a  particular  case. 
These  tests  are  of  two  kinds:  i.  Determination  of 
the  mineral  composition  through  petrographic  analysis. 
2.  Tests  of  the  physical  properties  of  road  materials. 

PETROGRAPHIC   ANALYSIS. 

The  following  methods  of  examination  have  been 
used  by  the  Office  of  Public  Roads  of  the  U.  S.  Agri- 
cultural Department,  and  are  described  by  Mr.  Edwin 
C.  E.  Lord  in  Bulletin  No.  31,  August,  1907. 

"Upon  receipt  of  the  rock  sample,  which,  accord- 
ing to  the  specification  of  this  Office,  should  weigh  not 


138       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

less  than  30  pounds  and  be  collected  with  care  to  repre- 
sent as  nearly  as  possible  an  average  of  the  whole 
exposure,  it  is  examined  in  a  general  way  to  determine 
the  proper  method  of  analysis. 

"  Rocks  consisting  essentially  of  the  carbonates  of 
lime  and  magnesia  (limestones,  dolomites,  etc.),  as 
well  as  fine-grained  shales  and  unconsolidated  sedi- 
mentary deposits,  such  as  clays,  sands,  gravels,  etc., 
are  analyzed  chemically  when  necessary,  whereas  all 
other  materials  are  prepared  for  microscopic  exami 
nation  or  determined  macroscopically. 

"The  mineral  composition  of  a  rock  may,  under 
favorable  conditions,  be  estimated  with  considerable 
accuracy  by  a  macroscopic  examination,  yet  for  exact 
quantitative  results  the  aid  of  a  polarizing  microscope 
and  transparent  thin  sections  of  the  rock  are  essential. 

Macroscopic  Method.  "The  macroscopic  form  ot 
analysis  can  be  applied  only  to  coarse-grained  rock,  in 
which  the  various  mineral  components  are  easily 
detected  with  the  unaided  eye.  The  approximate 
volumetric  relations  of  these  minerals  may  be  deter- 
mined by  preparing  a  smooth  surface  of  the  rock 
sample  and  covering  it  with  a  transparent  celluloid 
scale  divided  into  100  equal  square  areas  and  estimat- 
ing the  minerals  present  from  the  number  of  areas 
covered  by  each  mineral.  Any  properly  graduated 
scale  can  be  used,  but  a  transparent  one  is  preferable. " 

Microscopic  Methods.  "Owing  to  the  large  amount 
of  material  received  in  this  laboratory  it  has  been 
found  necessary  to  perfect  a  more  rapid  method  of 
quantitative  analysis  than  any  hitherto  described. 

"The  laboratory  is  equipped  with  an  exceptionally 
good  petrographic  microscope  of  the  latest  Fuess  model, 
which,  beside  the  usual  attachments,  is  provided  with 


BROKEN-STONE  ROADS.  139 

a  revolving  analyzer  in  the  tube  to  aid  in  the  deter- 
mination of  very  low  double  refracting  minerals,  and 
a  Schwarzmann  scale  for  the  measurement  of  optical 
axial  angles. 

"Another  important  accessory  is  a  detachable 
screw-micrometer,  movable  in  the  focal  plane  of  the 
ocular  by  means  of  a  drum  screw,  which,  with  the 
most  powerful  objective  (one-twelfth-inch  oil  immer- 
sion), records  a  drum-interval  of  0.00004  mm.  The 
measuring  apparatus  devised  by  Mr.  L.  W.  Page  and 
used  for  the  mineral  determinations  consists  of  an 
ordinary  fixed  eyepiece  having  a  square  field  divided 
into  100  quadratic  areas.  With  the  aid  of  this  cross- 
line  field,  each  square-  of  which  is  one  one-hundredth 
of  the  whole  field,  the  relative  proportions,  expressed 
in  per  cent,  of  the  minerals  occupying  the  field  can  be 
readily  determined  by  simply  noting  the  number  of 
squares  covered  by  each  mineral  in  turn.  Averages 
derived  from  numerous  examinations  of  this  kind  in 
various  parts  of  the  section  indicate  the  percentage  of 
the  different  minerals  constituting  the  rock  itself. " 

"Experience  has  shown  that  with  a  large  majority 
of  rock  samples  twenty  determinations,  using  a  magni- 
fication of  52  diameters,  give  very  satisfactory  results. 
In  the  case  of  extremely  fine-grained  rocks,  however, 
it  is  best  to  use  a  three-quarter-inch  objective  lens 
which  enlarges  105  diameters  when  combined  with  the 
eyepiece  micrometer. 

"With  rocks  having  an  average  grain  exceeding 
5  mm.,  or  those  varying  greatly  in  texture,  as  in  the 
case  of  porphyritic  and  schistose  varieties,  it  is  in 
some  instances  well  to  employ  a  two-inch  objective  in 
combination  with  an  ocular  prepared  in  the  same 
manner  as  that  just  described,  but  divided  into  only 


140       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

25  square  areas  and  magnifying  30  diameters.  In 
the  case  of  these  exceptionally  coarse-grained  rocks, 
two  or  more  thin  sections  of  the  same  sample  are 
examined  before  reliable  results  can  be  obtained. " 


PHYSICAL   TESTS. 

The  physical  properties  of  stone  for  road-building 
are  commonly  tested  by  determining  the  percentage 
of  wear,  using  the  Deval  abrasion  apparatus,  and  the 
cementation  properties  by  the  use  of  the  Page  cementa- 
tion test.  Tests  are  also  sometimes  made  of  the  crush- 
ing strength,  resistance  to  impact,  and  resistance  to 
abrasion  by  grinding,  and,  in  some  instances,  the 
specific  gravity  and  absorption  of  the  rock  are  deter- 
mined. 

Abrasion  Test.  This  test  was  first  used  in  France, 
and  is  commonly  known  as  the  Deval  test,  bearing  the 
name  of  its  designer.  The  Deval  machine  consists  of 
cylinders  20  cm.  in  diameter  and  34  cm.  in  depth, 
closed  at  one  end  and  with  a  tightly  fitting  cover  for 
the  other.  Two  or  four  of  these  cylinders  are  mounted 
upon  a  horizontal  shaft  so  that  the  axis  of  each  cylinder 
is  inclined  at  an  angle  of  30  degrees  with  the  axis  of 
rotation. 

The  method  of  conducting  the  test  in  the  investiga- 
tions of  the  U.  S.  Office  of  Public  Roads  is  as  follows:  * 
"The  sample  to  be  tested  is  first  broken  in  pieces  that 
will  pass  in  all  positions  through  a  6  centimeter  (2.4 
inch)  ring.  The  stones  are  then  cleansed,  dried  in  a 
hot-air  bath  at  100  degrees  C.,  and  cooled  in  a  desiccator. 
Five  kilograms  are  weighed  and  placed  in  one  of  the 
cylinders,  the  cover  bolted  on,  and  the  machine  rotated 

*Bureau  of  Chemistry,  Bulletin  No.  79. 


BROKEN-STONE  ROADS.  141 

at  the  rate  of  2000  revolutions  per  hour  for  5  hours. 
When  the  10,000  revolutions  of  the  machine  are  com- 
pleted the  contents  of  the  cylinder  are  placed  on  a 
sieve  of  0.16  centimeter  (TV  inch)  mesh,  and  the 
material  which  passes  through  is  again  sifted  through 
a  sieve  of  0.025  centimeter  (o.oi  inch)  mesh.  Both 
sieves  and  the  fragments  of  rock  remaining  on  them 
are  held  under  running  water  till  all  adhering  dust  is 
washed  off.  After  the  fragments  have  been  dried  at 
100  degrees  C.  and  cooled  in  a  desiccator  they  are 
weighed,  and  their  weight  subtracted  from  the  original 
5  kilograms  (n  pounds).  The  difference  obtained 
is  the  weight  of  the  detritus  under  0.16  centimeter 
(^  inch)  worn  off  in  the  test." 

In  the  French  experiments  it  was  found  that  the 
best  grades  of  rock  gave  about  20  grams  of  detritus 
per  kilogram  of  rock  tested,  and  the  number  20  was 
adopted  as  a  standard  and  the  "coefficient  of  wear" 
determined  from  the  formula: 

20       400 

Coefficient  of  wear  =  20  X  -  —  ' 

W        W 

in  which  W  is  the  weight  in  grams  of  detritus  under 
o.i 6  centimeter  (^  inch)  in  size  obtained  per  kilo- 
gram (2.2  pounds)  of  stone.  The  French  coefficient 
is  sometimes  used  in  stating  results  in  American  tests, 
but  it  is  more  common  to  ues  the  "percentage  of 
wear,"  which  is  found  by  stating  the  weight  of  detritus 
under  0.16  centimeter  in  terms  of  percentage  of  the 
weight  of  rock  tested.  In  this  case, 

4° 

Percentage  of  wear  =  - — — — —    — —  —  • 

Coefficient  of  wear 

In  some  of  the  work  of  the  United  States  Agricul- 
tural Department  another  coefficient,  known  as  the 


142       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

United  States  Agricultural  Department  coefficient  of 
wear,  has  been  employed.  This  coefficient  is  found  by 
subtracting  4000  grams  from  the  weight  of  the  frag- 
ments over  3  centimeters  (1.2  inches)  which  remain 
after  the  test  and  dividing  the  difference  by  10.  By 
this  method,  if  20  per  cent  of  the  material  is  abraded 
from  the  original  5000  grams,  the  coefficient  is  0  and 
the  material  considered  worthless;  if  no  dust  is  worn 
off,  the  coefficient  is  1 00. 

The  Committee  on  Standard  Tests  for  Road  Materials, 
of  the  American  Society  for  Testing  Materials,  in  1904 
recommended  the  following  specification  for  the 
abrasion  test:  "The  machine  shall  consist  of  one  or 
more  hollow  iron  cylinders,  closed  at  one  end  and 
furnished  with  a  tightly  fitting  iron  cover  for  the 
other;  the  cylinders  to  be  20  centimeters  in  diameter 
and  34  centimeters  in  depth  inside.  These  cylinders 
are  to  be  mounted  on  a  shaft  at  an  angle  of  30  degrees 
with  the  axis  of  rotation  of  the  shaft. 

"  At  least  30  pounds  of  coarsely  broken  stone  should 
be  available  for  a  test.  The  rock  to  be  tested  should 
be  broken  in  pieces  as  nearly  uniform  as  possible,  and 
as  nearly  50  pieces  as  possible  shall  constitute  a  test 
sample.  The  total  weight  of  rock  in  a  test  should  be 
within  10  grams  of  5  kilograms.  All  test  pieces  should 
be  washed  and  thoroughly  dried  before  weighing; 
10,000  revolutions  at  the  rate  of  between  30  and  33  to 
the  minute,  must  constitute  a  test.  Only  the  per- 
centage of  material  worn  off  which  will  pass  through 
a  O.I 6  centimeter  mesh  sieve  should  be  considered 
in  determining  the  amount  of  wear.  This  may  be 
expressed  either  in  the  per  cent  of  the  5  kilograms 
used  in  the  test,  or  the  French  coefficient,  which  is  in 
more  general  use,  may  be  given." 


BROKEN-STONE   ROADS.  143 

Cementation  Test.  This  test  was  developed  by 
Mr.  Logan  Waller  Page  while  geologist  of  the  Mas- 
sachusetts Highway  Commission.  It  consists  in  grind- 
ing the  stone  into  dust,  wetting  and  moulding  the 
dust  into  a  small  cylinder,  which  is  dried  and  then 
tested  by  subjecting  it  to  the  impact  of  the  falling 
weight.  The  method  of  conducting  this  test  as  used 
by  the  Office  of  Public  Roads  of  the  United  States 
Department  of  Agriculture  is  as  follows:  "One 
kilogram  of  the  rock  to  be  tested  is  broken  sufficiently 
small  to  pass  a  6  millimeter  but  not  a  I  millimeter 
screen.  It  is  then  placed  in  a  ball  mill  and  is  ground 
for  two  hours  and  a  half.  This  ball  mill  contains  two 
chilled  iron  balls  which  weigh  25  pounds  each,  and  is 
revolved  at  the  rate  of  2000  revolutions  per  hour.  It 
was  found  by  experiment  that  grinding  rock  thus  pre- 
pared for  two  hours  and  a  half  \vas  sufficient  to  reduce 
it  to  a  powder  that  would  pass  through  a  0.25  milli- 
meter mesh.  The  dust  thus  obtained  is  mixed  with 
water  to  about  the  consistency  of  a  stiff  dough,  and 
is  kept  in  a  closed  jar  for  twenty-four  hours.  About 
25  grams  of  this  dough  is  placed  in  a  cylindrical  metal 
die  25  millimeters  in  diameter.  A  closely  fitting  plug, 
supported  by  guide  rods,  is  inserted  over  the  material, 
which  is  then  subjected  to  a  pressure  of  1 00  kilograms 
per  square  centimeter. 

"  It  is  most  important  that  these  briquettes  should 
be  compressed  in  a  uniform  manner,  and  for  this  a 
special  machine  has  been  designed.  The  die  is  placed 
on  an  iron  platform  supported  by  a  piston  rod,  which 
is  connected  directly  with  a  hydraulic  piston  below. 
Water  from  a  tank  is  admitted  to  the  hydraulic  cylinder 
through  a  small  orifice  in  the  pipe.  As  the  piston 
rises  the  platform  and  die  are  carried  up  with  it,  the 


144       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS* 

plug  of  the  latter  coming  in  contact  with  a  yoke 
attached  to  a  properly  weighted  lever  arm.  When  the 
lever  arm  is  raised  one-eighth  of  an  inch  it  closes  an 
electric  circuit  which  trips  a  right  angle  cock,  shutting 
off  the  water  and  opening  the  exhaust.  One  minute  is 
required  to  compress  a  briquette,  and  the  maximum 
load  is  applied  only  for  an  instant.  By  this  device 
practically  uniform  conditions  are  obtained. 

"The  height  of  the  briquette  is  measured,  and  if  it 
is  not  exactly  25  millimeters  the  required  amount  of 
material  is  added  or  subtracted  to  make  the  next 
briquette  the  required  height.  Five  briquettes  are 
made  from  each  test  sample,  and  allowed  to  dry 
twelve  hours  in  air  and  twelve  hours  in  a  steam  bath. 
After  cooling  in  a  desiccator  they  are  tested  by  impact 
in  a  machine  especially  designed  for  the  purpose/' 

The  machine  commonly  used  for  this  purpose  is 
known  as  the  Page-Johnson  Impact  Machine.  It  was 
designed  by  Mr.  L.  W.  Page  and  afterward  modified 
by  Mr.  A.  N.  Johnson.  The  blow  is  delivered  by  a 
hammer  weighing  one  kilogram  striking  upon  a 
flat -end  plunger,  which  is  pressed  upon  the  briquette 
by  two  light  spiral  springs.  The  standard  fall  of  the 
hammer  for  a  test  is  I  centimeter  (0.39  inch),  and 
this  blow  is  repeated  until  the  bond  of  cementation  of 
the  material  is  destroyed.  The  number  of  blows 
required  is  noted  and  the  average  obtained  upon  five 
briquettes  is  given  as  the  cementing  value. 

In  making  this  test  the  results  may  be  considerably 
affected  by  slight  differences  in  manipulating  the 
material.  It  is  important  that  the  same  amount  of 
kneading  be  used  in  all  tests  and  that  the  dough  should 
be  allowed  to  stand  at  least  24  hours  before  forming 
the  cylinders. 


BROKEN-STONE   ROADS.  145 

Grinding  Test.  The  test  for  abrasion  by  grinding 
is  sometimes  used  in  France,  where  it  is  known  as  the 
Dorry  test.  It  has  also  been  used  by  the  Office  of 
Public  Roads  at  Washington.  The  object  of  the  test 
is  to  give  a  measure  of  the  hardness  of  the  rock.  It 
gives  interesting  information  concerning  the  material, 
but  is  not  of  special  value  in  testing  road  material. 
The  test  is  made  as  follows:  "The  test  piece  in  the 
form  of  a  cylinder  about  3  inches  in  length  by  I  inch 
in  diameter  is  prepared  by  an  annular  core  drill  and 
placed  in  the  grinding  machine  in  such  a  manner  that 
the  base  of  the  cylinder  rests  on  the  upper  surface  of 
a  circular  grinding  disk  of  cast  iron,  which  is  rotated 
in  a  horizontal  plane  by  a  crank  movement.  The 
specimen  is  weighted  so  as  to  exert  a  pressure  of 
250  grams  per  square  centimeter  against  the  disk, 
which  is  fed  from  a  funnel  writh-  sand  of  about  I  ^ 
millimeters  in  diameter.  After  1000  revolutions  the 
loss  in  weight  of  the  sample  is  determined  and  the 
coefficient  of  wear  obtained  by  deducting  one-third 
of  this  loss  from  20." 

Impact  Test.  This  test  is  intended  as  a  measure  of 
the  toughness  of  the  material.  It  is  frequently  made, 
although  not  of  special  value  as  a  test  for  road  material. 
It  is  made  as  follows:  "The  test  piece  is  a  cylindrical 
rock  core  similar  to  that  used  in  determining  hardness 
and  the  test  is  made  with  an  impact  machine  con- 
structed on  the  principle  of  the  pile  driver.  The  blow 
is  delivered  by  a  hammer  weighing  2  kilograms,  which 
is  raised  by  a  sprocket  chain  and  released  automatically 
by  a  concentric  electro-magnet.  The  test  consists  of  a 
I  centimeter  fall  of  the  hammer  for  the  first  blow  and 
an  increased  fall  of  I  centimeter  for  each  succeeding 
blow  until  the  failure  of  the  test  piece  occurs.  The 


146       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

number  of  blows  required  to  cause  this  failure  repre- 
sents the  toughness. " 

The  abrasion  and  cementation  tests  are  frequently 
employed  for  the  purpose  of  comparing  the  properties 
of  various  road  stones,  and  afford  a  means  by  which  a 
judgment  may  be  formed  as  to  the  probable  relative 
values  of  various  materials  for  road  construction.  No 
fixed  standard  for  comparison  has  been  devised,  and  the 
relative  importance  of  the  various  properties  depends 
upon  the  character  of  the  road  to  be  constructed. 

ART.  38.     ROAD  METAL. 

Stone  is  prepared  for  use  in  road  work  by  crushing  and 
screening.  In  the  early  days  of  broken-stone  roads, 
all  stone  was  broken  by  hand,  and  the  roads  were  care- 
fully constructed  of  stone  broken  to  approximately  uni- 
form sizes  without  the  addition  of  a  binding  material. 
The  development  of  stone  crushing  machinery  has, 
however,  modified  practice  in  this  regard  and  stone 
crushed  by  machinery  is  now  almost  exclusively  used. 
It  gives  satisfaction  both  as  to  binding  properties  and 
durability,  and  has  the  advantage  of  greatly  lessening 
the  cost. 

The  size  to  which  stone  should  be  broken  for  road 
material  depends  to  some  extent  upon  the  nature  of 
the  material.  The  harder  and  tougher  it  is  the  smaller 
the  pieces  may  be  without  danger  of  crushing  or  shat- 
tering under  the  loads  and  shocks  received  in  the  road 
surface,  and  the  smaller  also  the}^  will  need  to  be  in 
order  to  be  thoroughly  compacted  in  the  road. 

There  is  a  difference  of  opinion  among  roadbuilders 
as  to  the  advisability  of  using  stone  of  uniform  size. 
Some  insist  quite  strenuously  upon  this  point  and  care- 


BROKEN-STONE  ROADS.  147 

fully  screen  their  stone  with  the  object  of  getting  it  as 
uniform  as  possible;  while  others  declare  that  the  varia- 
tion of  size  is  an  advantage,  and  even  that  the  stone 
should  not  be  screened  after  coming  from  the  crusher, 
except  to  remove  the  stone  above  the  limiting  size  and 
to  get  rid  of  dust  and  foreign  matter. 

Uniformity  of  size  probably  makes  the  wear  more 
even,  but  the  presence  of  smaller  fragments  facilitates 
the  binding  together  of  the  material.  If  the  varying 
sizes  bs  well  distributed  through  the  mass  of  stone,  the 
variation  of  size  has  the  advantage  of  lessening  the  amount 
of  voids,  and  makes  it  possible  to  compact  the  stone 
in  the  road  with  a  less  quantity  of  binder.  Screening 
out  the  fine  parts  and  dust  eliminates  the  danger  of  having 
portions  of  the  road  made  up  entirely  of  fins  material, 
and  secures  a  proper  distribution  of  the  binder  through 
the  mass  of  stone. 

The  lower  courses  of  stone  in  the  road  may  be  of  any 
sizes  which  are  most  convenient,  provided  the  stones 
are  not  too  large  to  become  firmly  compacted  under  the 
roller.  When  the  stones  of  the  surface  layer  are  small 
in  size,  it  is  common  to  use  the  larger  sizes  in  the  bottom 
course,  thus  making  it  unnecessary  to  break  all  to  the 
small  dimension. 

If  the  surface  of  a  road  is  to  be  constructed  of  very 
hard  rock  the  stones  for  the  surface  layer  may  include 
those  from  about  J  inch  to  i^  inches  (or  at  most  ij  inches) 
in  diameter  with  good  results.  In  the  work  of  the 
Massachusetts  Highway  Commission:  "  All  broken  stone 
used  is  separated  into  three  sizes  by  passing  it  through  a 
screen  with  meshes  J  inch,  ij  inches,  and  2\  inches  in 
diameter.  The  largest  size  is  placed  at  the  bottom  and  is 
covered  with  the  successive  smaller  sizes.  The  different 
sizes  of  stone  are  spread  in  courses.  The  sub-grade  and 


148       A  TEXT-BOOK  ON  ROADS   AND   PAVEMENTS. 

each  course  of  stone  are  rolled  thoroughly,  and  the  top 
course  is  watered  before  rolling." 

In  constructing  roads  with  limestones,  it  is  often  desir- 
able to  use  larger  materials  for  the  surface  layer,  as  these 
offer  better  resistance  to  the  wear  of  traffic.  For  such 
roads  the  surface  may  be  composed  of  stones  from  about 
ij,  or  if  inches  to  3,  or  even  3^  inches  in  diameter;  the 
smaller  sizes  being  used  in  the  bottom  course  of  the  road, 
and  tho  screenings  for  binder.  Stone  of  this  character 
may  also  be  advantageously  used  by  making  both  courses 
of  material  containing  a  greater  range  of  sizes,  as  in  the 
specifications  of  the  Illinois  Highway  Commission  for 
1911,  which  require  that: 

"  Sizes:  Two  sizes  of  crushed  stone  shall  be  used  as 
follows:  (a)  Broken  to  a  size  that  will  pass  over  a  i-inch 
ring,  and  through  a  2j-inch  ring,  which  size  will  herein- 
after be  referred  to  as  2  J-inch  stone,  (b)  Broken  to  a  size 
that  will  pass  through  a  i-inch  ring  and  graded  to  a  dust, 
which  size  will  hereinafter  be  referred  to  as  screenings. 

"  The  first  course  of  stone  shall  be  2j-inch  broken 
stone  spread  to  compact  under  rolling  to  the  thickness 
shown  on  the  plans. 

"  After  the  first  course  of  stone  has  been  spread,  it 
shall  be  harrowed  with  a  stiff-tooth  harrow  (having  a 
weight  of  10  to  12  pounds  per  tooth)  until  a  uniform  size 
of  stone  is  brought  to  the  surface  and  all  fine  material 
which  may  have  been  mixed  with  the  2j-inch  stone  has 
been  shaken  to  the  bottom  of  the  layer  of  stone. 

"  After  the  broken  stone  for  the  first  course  has  been 
spread  to  a  uniform  thickness  and  harrowed,  and  has  a 
proper  cross-section,  it  is  to  be  rolled  with  a  steam  roller 
weighing  not  less  than  10  tons,  until  it  is  compacted 
to  form  a  firm,  smooth  surface.  The  rolling  must 
begin  at  the  sides  and  work  towards  the  center  and 


BROKEN-STONE  ROADS.  149 

the  rear  wheels  of  the  roller  must  cover  this  space 
thoroughly." 

The  second  course  of  stone  is  spread  in  the  same  manner 
as  the  first,  after  which  the  filler  is  applied,  as  follows: 

"  After  the  second  course  of  stone  has  been  rolled  and 
completed  as  specified,  the  screenings  are  to  be  spread, 
but  in  no  case  are  screenings  to  be  used  until  the  second 
course  has  been  thoroughly  rolled  and  compacted.  The 
screenings  are  to  be  spread  dry  with  shovels  from  piles 
along  the  road,  or  from  dumping  boards,  but  in  no  case 
are  the  screenings  to  be  dumped  directly  on  the  second 
course.  The  quantity  of  screenings  used  is  to  be  such 
as  will  just  cover  the  second  course  of  stone. 

"After  the  screenings  are  spread  they  are  to  be  sprinkled 
with  water  from  a  properly  constructed  sprinkling  cart 
and  then  rolled  with  a  steam  roller  weighing  not  less 
than  10  tons.  The  amount  of  water  to  be  used  to  be 
determined  by  the  engineer.  The  rolling  is  to  begin 
at  the  sides  and  to  continue  until  the  surface  is  hard  and 
smooth  and  shows  no  perceptible  tracks.  The  rolling 
and  watering  shall  continue  until  the  water  flushes  to 
the  surface. 

"  If  after  rolling  the  screenings,  the  stone  appears 
at  the  surface,  additional  screenings  shall  be  used  in  such 
places.  The  rolling  is  to  extend  over  the  whole  width 
of  the  macadam." 

Gravel  is  frequently  used  for  roads  constructed  in 
the  same  manner  as  with  broken  stone,  both  with  and 
without  the  telford  foundation.  The  requirements  of 
a  good  gravel  for  this  purpose  are  the  same  as  for  a 
good  stone.  The  stones  of  the  gravel  should  be  sharp 
and  angular,  and  must  possess  the  qualities  of  hardness 
and  toughness.  Water-worn  material  is  therefore  ob- 
jectionable, as  it  will  not  compact  without  the  use  of 


150       A  TEXT-BOOK   ON  ROADS   AND   PAVEMENTS. 

large  amounts  of  soft  binding  material.  In  many 
places  a  hard  flint  gravel  occurs  which  is  not  inferior  to 
the  best  broken  stone.  This  frequently  occurs  when 
the  available  rock  is  soft  limestone  and  may  be  used 
to  advantage  as  a  surface  upon  a  base  of  the  soft  rock. 

Gravel  should  be  screened  to  remove  the  larger  stones 
and  the  fine  material,  and  then  treated  in  the  same 
manner  as  broken  stone. 

Gravel  not  fit  for  surface  material  may  often  be  used 
to  advantage  as  a  base  under  a  surface  of  hard  rock; 
in  many  instances,  economy  would  result  from  the 
substitution  of  gravel  for  broken  stone  in  such  work. 
Slag  and  cinders  may  also  sometimes  be  used  in  the 
same  manner. 

In  cases  where  local  stone  is  being  used  for  the  lower 
courses  of  a  road  to  be  surfaced  with  trap  or  other 
more  durable  rock,  or  where  a  flint  gravel  surface  is 
used  upon  a  soft  limestone  base,  the  screenings  from 
the  stone  used  in  the  lower  layer  may  often  advan- 
tageously be  used  in  binding  the  surface,  the  whole 
run  of  the  crusher  except  the  screenings  through  about 
a  one-half  inch  screen  being  used  in  the  lower  course  of 
the  road. 

In  some  instances  the  binding  material  is  mixed  with 
the  surface  stone  before  placing  upon  the  road.  The 
following  extract  from  the  specifications  of  Mr.  James 
Owen  for  roads  in  Essex  County,  New  Jersey,  shows 
this  practice:  "When  the  two  courses  are  rolled  to 
the  satisfaction  of  the  Engineer  and  Supervisor,  a  coat 
of  fifty  (50)  per  cent  of  three-quarters  (f)  inch  stone 
and  fifty  (50)  per  cent  of  screenings  properly  mixed 
is  to  be  spread  of  sufficient  thickness  to  make  a  smooth 
and  uniform  surface  to  the  road;  then  again  rolled 
until  the  road  becomes  thoroughly  consolidated,  hard 


BROKEN-STONE   ROADS.  151 

i 

and  smooth. "  This  specification  is  remarkable  for 
the  large  quantity  of  screenings  used,  and  needs  great 
care  in  securing  a  proper  mixture  of  the  two  materials. 

ART.  39.     COMPACTING  THE  ROAD. 

The  materials  may  be  compacted  in  a  road  either  by 
placing  them  in  position  and  allowing  the  traffic  to 
pass  over  them  or  by  rolling  with  a  steam  or  horse 
roller. 

The  first  method  by  itself  is  seldom  practiced  when 
it  is  possible  to  avoid  it.  It  is  hard  upon  the  traffic, 
takes  a  long  time  to  reduce  the  road  to  compact  con- 
dition, and  a  smooth  surface  is  with  difficulty  pro- 
duced. Where  heavy  horse  rollers  are  employed  they 
are  clumsy  and  inconvenient  to  handle,  and  the  work 
of  rolling  is  slow  as  compared  with  the  steam  roller. 
In  many  instances,  however,  good  results  are  obtained 
with  them.  They  are  not  so  expensive  in  first  cost  as 
steam  rollers,  and  have  not  the  disadvantage  of  fright- 
ening horses. 

Horse  rollers  are  usually  arranged  so  that  the  direc- 
tion of  motion  may  be  reversed  without  turning  the 
roller  itself  around,  and  also  so  that  the  weight  may  be 
changed  by  placing  additional  weight  inside  the  roller 
or  removing  it.  Horse  rollers  for  this  purpose  usually 
bring  a  pressure  of  from  125  to  250  pounds  per  linear 
inch  upon  the  road  and  weigh  from  3  to  6  tons. 

Steam  rollers  weighing  from  8  to  15  tons  are  most 
commonly  employed  for  compacting  the  road  mate- 
rials. They  have  the  advantage  of  forcing  the  materials 
at  once  into  a  firm  and  compact  mass  and  producing  a 
smooth  surface  for  the  immediate  use  of  travel.  Thev 


152      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

admit  also  of  the  use  of  hard  materials  for  binding. 
These  rollers  give  a  pressure  under  the  drivers  of  from 
400  to  650  pounds  per  linear  inch. 

The  stone  forming  the  body  of  the  road  should  be 
placed  and  partially  compacted  before  the  addition  of 
the  small  material,  which  may  then  be  worked  into  the 
spaces  between  them. 

The  office  of  the  binding  material  is  to  hold  the 
stones  in  place  and  form  a  bearing  for  them,  as  well  as 
to  prevent  the  passage  of  water  between  them.  It  has 
no  duty  to  perform  in  sustaining  the  loads.  This  is 
the  objection  to  having  the  binding  material  mixed 
with  the  stones  in  advance,  as  would  be  the  case  when 
unscreened  stone  is  used.  A  portion  of  the  road  stones 
would  be  replaced  by  small  material  instead  of  having 
this  material  only  in  such  voids  as  necessarily  exist  be- 
tween the  stones. 

The  quantity  of  binding  to  be  used  is  that  which 
will  be  barely  sufficient  to  fill  all  the  voids  in  the  larger 
material.  It  has  been  contended  that  the  lower  por- 
tion of  the  road  should  be  porous  in  order  to  facilitate 
the  escape  of  any  water  that  may  find  its  way  through 
the  surface,  but  the  tendency  of  the  best  modern  prac- 
tice is  in  the  direction  of  filling  all  the  voids  as  nearly 
as  possible,  thus  making  the  entire  road  practically  one 
solid  body,  and  it  is  now  commonly  agreed  that  the  sur- 
face of  a  properly  constructed  broken-stone  road  is  very 
nearly  impervious  to  water. 

The  voids  in  loose  broken  stone  comprise  about  40 
to  50  per  cent  of  the  volume.  In  the  stone  when 
compacted  in  the  road  the  voids  are  somewhat  reduced, 
probably  ranging  from  30  to  40  per  cent  of  the  volume. 
The  voids  may  be  approximately  determined  in  any 


BROKEN-STONE  ROADS.  153 

case  by  filling  a  measure  with  the  stone,  shaken  down 
as  closely  as  possible,  and  then  measuring  the  quantity 
of  sand  that  can  be  added  in  the  same  manner. 

In  constructing  a  road  with  the  use  of  a  steam-roller, 
the  road-stone  is  first  put  on  to  the  required  thickness 
and  the  roller  passed  over  it  to  settle  the  stones  into 
place  and  reduce  the  voids  as  much  as  possible.  The 
binding  material,  representing  a  volume  about  equal  to 
the  voids  in  the  stone,  is  then  added,  sprinkled,  and 
rolled  until  the  small  material  is  washed  and  forced  into 
the  interstices,  giving  a  smooth,  hard  surface.  This  is 
repeated  for  each  layer  of  stone,  or  in  some  cases  the 
small  material  is  applied  only  to  the  top  layer. 

A  thin  coating  of  the  binding  material  is  then  spread 
upon  the  surface  and  the  road  thrown  open  for  travel. 

ART.  40.    THICKNESS  OF  ROAD-COVERING. 

The  thickness  necessary  for  a  road-covering  depends 
upon  the  amount  of  the  traffic  it  is  to  bear  and  upon  the 
nature  of  the  foundation  afforded  by  the  road-bed. 
Under  a  heavy  traffic  it  is  advisable  to  make  the  road- 
covering  heavier  than  might  be  allowable  for  lighter 
traffic,  in  order  to  provide  for  wear  and  lessen  cost  of 
renewals. 

When  the  road-bed  is  firm,  well  drained,  and  not 
likely  to  soften  at  a  wet  season,  it  will  always  afford  a 
firm  bearing,  upon  which  the  covering  may  rest.  The 
loads  coming  upon  the  road  are  then  simply  transmitted 
through  the  covering  to  the  road-bed  beneath,  and 
there  is  no  tendency  on  the  part  of  the  loads  to  break 
through  the  covering  other  than  by  direct  crushing  of 
its  material.  If,  however,  the  road-bed  may  become 
soft  in  wet  weather,  it  will  then  lose  its  power  to  firmly 


154        A  TEXT-BOOK  ON  ROADS   AND   PAVEMENTS. 

sustain  the  covering  at  all  points,  and  the  covering 
must  possess  sufficient  strength  to  bridge  over  places 
where  it  is  not  supported  from  beneath,  or  a  load  com- 
ing upon  it  may  break  through  by  bending  it  down- 
ward at  such  point.  The  thickness  of  road-covering, 
therefore,  must  be  greater  where  the  road-bed  is  less 
perfect. 

The  intensity  of  freezing  that  may  be  expected  also 
has  an  influence  upon  the  necessary  thickness  of  the 
road-covering.  The  effect  of  frost  upon  the  road  will 
depend  in  large  measure  upon  the  condition  of  the 
road-bed,  and  thus,  make  the  thickness  depend  in  still 
greater  measure  upon  its  nature.  Freezing  will  not 
injure  a  dry  road-bed,  but  if  it  be  damp  and  have  but 
a  thin  covering  the  road  is  likely  to  blow  or  be  thrown 
up  by  the  action  of  frost. 

For  roads  on  considerable  grades  the  thickness  of  the 
road-covering  is  often  reduced  below  what  is  used  on 
flat  ones,  because  of  the  better  drainage  afforded  by 
the  slopes.  It  is  to  be  remarked,  however,  that  if  the 
slopes  are  very  steep  the  wear  of  the  surface  becomes 
so  great,  due  to  the  horses'  efforts  to  obtain  foothold 
and  to  the  washing  of  surface-waters  during  rains, 
that  the  thickness  of  the  coating  should  be  increased. 

Macadam  roads  are  commonly  made  from  4  to  12 
inches  thick,  and  telford  roads  from  8  to  12  inches,  of 
which  5  to  8  inches  may  be  foundation  pavement. 

A  covering  6  to  8  inches  thick  is  usually  sufficient  for 
nearly  any  case  of  a  country  road,  unless  laid  upon  bad 
foundation,  or  to  carry  exceptionally  heavy  traffic. 
When  the  road-bed  is  formed  of  firm  material  and  well 
drained,  a  covering  of  4  or  5  inches  of  broken  stone  or 
gravel  may  give  good  service  under  considerable  traffic. 

A  thin  road  to  be  effective  must  have  its  interstices 


BROKEN-STONE   ROADS  155 

well  filled  with  binding  material  and  be  thoroughly 
compacted  by  rolling.  It  will  then  present  no  voids  to 
be  filled  by  the  soil  pressing  upward  from  below,  and 
at  the  same  time  it  will  be  practically  impervious  and 
prevent  surface-water  from  reaching  the  road-bed,  thus 
keeping  the  material  in  good  condition  to  sustain  the 
loads.  The  4-inch  roads  of  Bridgeport,  Conn.,  which 
are  often  cited  as  examples  of  successful  work,  are  con- 
structed in  this  manner  of  exceptionally  good  mate- 
rial, In  other  cases  where  thin  roads  have  proved  fail- 
ures the  trouble  may  often  be  traced  to  dampness  in 
the  subsoil  or  to  lack  of  thorough  construction. 

Instances  will  frequently  be  met  in  practice  where  a 
road  must  be  constructed  over  material  which  is  likely 
to  be  unstable  and  cannot  be  made  firm  by  drainage. 
In  such  cases,  thick  roads  must  be  built.  Where  the 
conditions  are  unfavorable,  a  road  12  to  1 6  inches  thick 
may  be  necessary. 

In  mam-  cases  the  problem  to  decide,  in  determining 
the  thickness  of  a  covering,  is  whether  to  use  heavy 
construction  or  thorough  drainage.  It  is  easier  to  get 
good  results  with  thick  road-coverings,  and  they  are 
in  general  safer  to  use;  but  skillful  adaptation  of  less 
material  may  often  save  expense  in  construction  \vith 
good  results.  The  peculiar  conditions  of  each  case  must 
decide  what  is  best  for  that  case. 

On  country  roads  the  macadam  surface  should  be 
given  a  crown  of  from  one-thirtieth  to  one-twent\^- 
fourth  of  the  width  in  order  to  provide  good  drainage. 
In  manyinstances  a  considerable  saving  in  road  material 
may  be  effected  by  making  the  road  thinner  at  the 
edges  than  in  the  middle.  The  Massachusetts  Highway 
Commission  in  some  instances  reduce  the  thickness  of 
their  6-inch  roads  to  2 J  or  3^  inches  at  the  edges. 


156      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Some  engineers  grade  the  road-bed  without  leaving  a 
bench  at  the  side,  and  reduce  the  stone  to  thin  edges. 
It  is  doubtful  if  there  is  any  economy  in  this  practice, 
as  it  is  wasteful  in  the  use  of  stone,  although  it  effects 
a  small  saving  in  the  cost  of  grading  the  road-bed. 

ART.  41.     MAINTENANCE  OF  BROKEN-STONE  ROADS. 

To  maintain  a  broken-stone  road  in  good  condition 
it  is  necessary  first  of  all  that  it  be  frequently  cleaned 
of  mud  and  dust,  and  that  the  gutters  and  surface 
drains  be  kept  open  to  insure  the  prompt  discharge 
of  all  water  that  may  come  upon  the  surface  of  the  road. 

The  best  method  of  making  repairs  that  may  become 
necessary  to  the  road-surface  depends  upon  the  char- 
acter of  the  material  composing  the  surface  and  the 
weight  of  the  traffic  passing  over  it. 

If  the  road  metal  be  of  soft  material  which  wears 
easily,  it  will  require  constant  supervision  and  small 
repairs  whenever  a  rut  or  depression  may  appear. 
Material  of  this  kind  binds  readily  with  new  material 
that  may  be  added,  and  may  in  this  manner  frequently 
be  kept  in  good  condition  without  great  difficulty,  while 
if  not  attended  to  at  once  when  wear  begins  to  show  it 
will  very  rapidly  increase,  to  the  great  detriment  of  the 
road.  In  making  repairs  by  this  method,  the  material 
is  commonly  placed  a  little  at  a  time  and  compacted  by 
the  traffic.  The  material  used  for  this  purpose  should 
be  the  same  as  that  of  the  road-surface,  and  not  fine 
material  which  would  soon  reduce  to  powder  under  the 
loads  which  come  upon  it.  By  careful  attention  to 
minute  repairs  in  this  manner  a  surface  may  be  kept 
in  good  condition  until  it  wears  so  thin  as  to  require 
renewal. 


BROKEN-STONE  ROADS.  157 

In  case  the  road  be  of  harder  material  that  will  not 
so  readily  combine  when  a  thin  coating  is  added,  the 
repairs  may  not  be  so  frequent,  as  the  surface  will  not 
wear  so  rapidly  and  immediate  attention  is  not  so 
important.  It  is  usually  more  satisfactory  in  this  case 
to  make  more  extensive  repairs  at  one  time,  as  a  larger 
quantity  of  material  added  at  once  may  be  more  readily 
compacted  to  a  uniform  surface,  the  repairs  taking  the 
form  of  an  additional  layer  upon  the  road. 

Where  the  material  of  the  road-surface  is  very  hard 
and  durable,  a  well-constructed  road  may  wear  quite 
evenly  and  require  very  little,  if  anything,  in  the  way 
of  ordinary  small  repairs  until  worn  out.  It  is  now 
usually  considered  the  best  practice  to  leave  such  a 
road  to  itself  until  it  wears  very  thin,  and  then  renew  it 
by  an  entirely  new  layer  of  broken  stone  placed  in  the 
same  manner  as  in  original  construction,  on  top  of  the 
worn  surface,  and  without  in  any  way  disturbing  that 
surface.  If  a  thin  layer  only  of  material  is  to  be 
added  at  one  time,  in  order  that  it  may  unite  firmly 
with  the  upper  layer  of  the  road  it  is  usually  necessary 
to  break  the  bond  of  the  surface  material  before  plac- 
ing the  new  layer,  either  by  picking  it  up  by  hand  or, 
if  a  steam  roller  is  in  use,  by  means  of  short  spikes  in 
its  surface.  Care  should  be  taken  in  doing  this,  how- 
ever, that  only  the  surface  layer  be  loosened,  and  that 
the  solidity  of  the  body  of  the  road  be  not  disturbed, 
as  might  be  the  case  if  the  spikes  are  too  long. 

Much  difficulty  has  also  been  experienced  in  some 
localities,  where  the  macadam  roads  connect  with  earth 
roads  which  in  wet  weather  are  composed  of  heavy, 
sticky  mud,  on  account  of  the  "  picking  up  "  of  the 
macadam  surface  in  muddy  weather  by  the  wheels 
of  vehicles  which  are  covered  with  mud.  The  stones 


158          A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

in  the  surface  are  loosened  and  carried  off  until  the  road 
is  destroyed.  This  has  given  much  difficulty  in  some  of 
the  states  of  the  middle  west,  where  limestone  macadam 
is  used.  This  trouble  is  reduced  by  forming  the  surface 
of  large  materials  but  considerable  strength  is  essential 
in  the  binding  material  to  successfully  resist  destruction 
from  this  cause. 

The  maintenance  of  macadam  roads  under  trying  condi- 
tions or  under  severe  traffic  has  in  many  instance*  proven 
a  matter  of  considerable  difficulty  and  of  large  expense. 
Under  ordinary  circumstances  the  destruction  of  a  broken 
stone  road  is  greatest  in  dry  and  dusty  weather.  If  the 
road  is  subject  to  considerable  travel,  wear  becomes 
rapid  and  a  certain  amount  of  the  road  metal  is  blown 
away  by  the  wind,  washed  away  in  case  of  rain,  or  cleaned 
from  the  surface  as  mud.  The  binding  material  wear- 
ing into  dust  and  being  removed  from  the  road  loosens 
the  stones  of  the  road-surface,  causing  the  road  to  "  ravel." 

To  protect  a  broken-stone  road  against  excessive 
wear  and  prevent  raveling  in  dry  weather,  some  means 
of  laying  the  dust  must  be  used.  Sprinkling  the  road- 
surface  with  water  is  often  used  for  this  purpose 
and  has  an  important  effect  in  reducing  the  wear  and 
prolonging  the  life  of  the  road.  If  the  road  be  sys- 
tematically sprinkled,  the  material  ground  off  by  the 
traffic  will  pack  upon  the  surface,  forming  a  cushion 
which  serves  to  protect  it  from  further  attrition.  In 
sprinkling,  the  object  should  be  to  keep  the  surface  damp, 
and  not  to  flood  it  by  applying  too  large  a  quantity  of 
water  at  once. 

The  recent  great  increase  in  the  extent  of  automobile 
travel  upon  country  roads  has  introduced  a  new  element 
into  the  problem  of  road  construction  and  maintenance. 
The  destructive  effect  of  these  rapidly  moving  vehicles 


BROKEN-STONE  ROADS.  159 

is  such  that,  under  any  considerable  traffic  of  this  char- 
acter, an  ordinary  macadam  road  is  quickly  destroyed, 
and  special  methods  of  construction,  or  of  maintenance, 
must  be  employed  if  satisfactory  results  are  to  be  obtained. 
The  driving  wheels  of  the  automobile  produce  a-  back- 
ward thrust  upon  the  surface  of  the  road  which  tends  to 
remove  the  binder  and  loosen  the  stones  composing 
the  surface,  while  the  rapid  motion  of  the  body  of  the 
car  causes  air  currents  which  draw  the  dust  from  the 
road  surface  and  throw  it  upward  behind  the  automobile. 
The  maintenance  of  country  roads  against  the  destruc- 
tive effect  of  automobile  travel  is  largely  a  question  of 
preventing  the  formation  of  dust  upon  the  surface  of 
the  road.  Sprinkling  with  water  may  serve  in  towns, 
but  is  expensive  and  not  applicable  on  country  roads. 
The  use  of  oil  in  earth-road  construction  has  been  dis- 
cussed in  Art.  29,  and  a  number  of  methods  have  been 
proposed  for  eliminating  dust  on  broken-stone  roads; 
these  will  be  discussed  in  Art.  42. 


ART.  42.    DUST  PREVENTION. 

The  excessive  production  of  dust  upon  country  high- 
ways of  large  traffic  presents,  to  the  road  engineer,  one 
of  the  most  important  problems  with  which  he  has  to 
deal.  This  dust  is  a  source  of  discomfort  to  people 
using  the  road,  or  living  in  its  vicinity,  a  menace  to  the 
health  of  those  breathing  the  dust-laden  air,  and  causes 
the  quick  destruction  of  the  surface  of  the  road  itself. 

The  discomforts  caused  by  road  dust  are  evident  to 
all  observers.  It  penetrates  into  the  houses,  damaging 
furnishings  and  causing  increased  labor  to  the  house- 
keeper, injures  clothing,  and  prevents  the  enjoyment 


160      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

of  lawns  and  porches.  Vegetation  in  the  vicinity  of  the 
road  is  often  injured  and  fruit  destroyed. 

The  effect  of  dust  upon  the  health  of  men  and  animals 
is  not  yet  fully  understood,  but  enough  is  known  to  indicate 
that  dust  is  an  efficient  agent  for  the  dissemination  of 
diseases  which  are  communicated  by  germs.  The 
elimination  of  dust  is  considered  of  special  importance 
in  the  effort  to  eradicate  tuberculosis. 

Road  dust  consists  of  very  finely  divided  particles  of 
material  abraded  from  the  surface  of  the  road  by  the 
traffic,  which,  when  in  a  dry  condition,  are  easily  carried 
away  by  the  winds  or  distributed  through  the  air  by 
passing  vehicles.  The  extent  to  which  dust  is  formed 
depends  therefore  upon  the  resistance  of  the  road  metal 
to  abrasion  and  the  tenacity  of  the  binder  used  to  hold 
the  stones  together,  as  well  as  upon  the  extent  to  which 
the  winds  and  traffic  remove  the  dust  from  the  road 
surface.  If  the  metal  abraded  by  the  traffic  would 
remain  upon  the  surface  of  the  road  it  would  serve  to 
protect  the  surface  from  further  abrasion,  and  prolong 
the  life  of  the  road  This  material,  however,  is  rapidly 
removed  as  dust  in  dry  weather,  or  washed  away  by 
rain,  leaving  the  surface  exposed  to  further  abrasion. 

METHODS    OF    DUST    PREVENTION. 

The  formation  of  dust  may  be  lessened  by  so  constructing 
the  road  as  to  cause  the  surface  to  offer  greater  resistance 
to  abrasion  and  shear  of  the  traffic,  or  by  moistening  the 
surface  so  as  to  prevent  the  particles  abraded  from 
flying  into  the  air,  and  cause  them  to  adhere  to  each  other 
and  to  readily  pack  upon  the  surface.  We  therefore 
distinguish  between  the  treatment  applied  to  ordinary 
macadam  surfaces  for  the  purpose  of  laying  dust,  and  the 


BROKEN-STONE  ROADS.  l6l 

construction  of  macadam  roads  with  special  binders, 
although  the  two  methods  have  much  the  same  objects 
in  view. 

The  construction  of  broken-stone  roads  with  special 
binders  will  be  discussed  in  a  separate  chapter  (see 
Chap.  VI)  under  the  head  of  Bituminous  Macadam. 
The  use  of  oil  upon  earth  roads  for  the  same  purpose 
has  been  discussed  in  Art.  29;  here  we  will  consider 
only  the  methods  used  for  laying  dust  upon  ordinary 
macadam  surfaces. 

Numerous  materials  have  been  proposed  for  use  as 
dust  layers  and  many  experiments  have  been  made  for 
the  purpose  of  determining  their  efficiency  in  use.  Com- 
paratively few  of  these. are  of  much  importance  and  in 
consequence  of  the  varying  conditions  under  which  the 
experiments  have  been  made  and  the  different  methods 
of  application,  it  is  somewhat  difficult  to  arrive  at  a 
definite  conclusion  as  to  the  relative  values  of  the  dif- 
ferent materials  or  the  best  way  of  applying  them. 

Water.  As  mentioned  in  Art.  41,  the  use  of  water  for 
sprinkling  the  surfaces  of  broken-stone  roads  not  only 
lays  the  dust,  but  has  a  marked  effect  upon  the  life  of 
the  road  by  protecting  the  surface  from  further  abrasion. 
The  use  of  water  for  this  purpose  has,  however,  the  dis- 
advantage of  requiring  frequent  sprinkling  on  account 
of  the  rapid  evaporation  of  the  water  in  dry  weather, 
and  is  on  this  account  not  readily  applicable  on  country 
roads,  where  water  is  not  easily  available.  For  the  streets 
of  towns,  sprinkling  is  still  very  commonly  employed, 
but  it  seems  probable  that  in  most  instances,  upon  macadam 
streets,  the  amount  of  labor  required  is  such  that  a  treat- 
ment of  more  permanent  character  would  be  cheaper. 

Calcium  Chloride.  The  use  of  calcium  chloride 
(CaCl2)  is  based  upon  its  hygroscopic  and  deliquescent 


1 62       A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

properties.  When  sprinkled  upon  the  road  surface  it 
retains  water,  or  absorbs  water  from  the  air,  and  liquefies, 
thus  keeping  the  surface  moist  and  preventing  the  abraded 
material  from  drying  into  dust. 

Calcium  chloride  may  be  obtained  either  in  a  solid 
granular  condition  or  in  a  concentrated  solution.  It 
is  produced  as  a  by-product  in  the  preparation  of  bicar- 
bonate of  soda  from  common  salt  by  the  use  of  ammonia. 
When  the  material  must  be  transported  to  considerable 
distances,  it  is  desirable  to  obtain  it  in  solid  form  on 
account  of  the  cost  of  transportation,  but  otherwise  the 
solution  is  convenient  as  saving  the  trouble  of  dissolving 
the  salt. 

In  applying  calcium  chloride  to  a  road  surface,  it  is 
customary  to  use  a  solution  containing  from  about  8  to 
20  per  cent  of  the  salt,  and  distribute  it  from  an  ordinary 
sprinkling  cart.  In  the  first  application,  the  more  con- 
centrated solution  (perhaps  15  per  cent)  may  be  used,  and 
when  other  applications  become  necessary  a  more  dilute 
solution  (8  per  cent  to  10  per  cent)  is  employed.  If  about 
600  gallons  of  the  solution  be  sprinkled  on  1000  feet  in 
length  of  an  18  feet  width  of  road,  from  15  to  j  pound  of 
the  calcium  chloride  is  required  per  square  yard  of  road 
surface  In  England,  somewhat  larger  quantities  than  this 
have  commonly  been  employed,  about  J  pound  being  used 
for  the  first  application,  with  subsequent  applications 
of  J  to  \  pound.  On  some  American  roads,  applications 
of  less  quantities  have  been  found  fairly  satisfactory, 
about  J  pound  being  used  for  the  first,  and  TO  or  J  pound 
for  subsequent  applications. 

When  the  calcium  chloride  is  obtained  in  solid  form  it 
should  be  dissolved,  forming  a  concentrated  solution  (40 
per  cent)  in  advance  of  the  time  when  it  is  to  be  used,  and 
placed  in  tanks  at  points  where  water  may  most  conven- 


BROKEN-STONE  ROADS.  163 

iently  be  obtained,  for  diluting  it,  near  the  road  to  be 
treated.  The  salt  is  readily  soluble  in  water.  If  several 
hours  may  be  allowed  for  the  dissolving  to  take  place, 
it  may  be  suspended  in  a  wire  basket  under  the  surface 
of  the  water  in  the  tank  until  solution  is  complete,  but, 
if  it  must  be  dissolved  for  immediate  use,  mechanical 
agitation  must  be  employed  to  hasten  the  process. 

In  the  humid  climate  of  England  this  method  has  been 
employed  for  laying  dust  with  good  results,  applications 
being  made  at  intervals  of  six  weeks  or  two  months  dur- 
ing the  summer  and  fall.  In  the  drier  climate  of  the 
United  States,  the  use  of  calcium  chloride  has,  in  some 
instances,  been  found  economical  as  a  substitute  for 
sprinkling  with  water,  although  it  has  been  found  neces- 
sary when  the  air  is  diy  to  supply  water  to  the  salt  by 
occasional  sprinkling,  sufficient  water  not  being  absorbed 
from  the  air  during  the  hot  part  of  the  day.  This  method 
is  not  applicable  to  ordinary  country  roads  in  the  United 
States,  and  can  only  be  used  at  reasonable  cost  where 
water  is  available  along  the  line  of  the  road  for  convenient 
use  in  sprinkling. 

The  application  of  calcium  chloride  has  no  permanent 
effect  on  the  road,  and  is  gradually  washed  away  by  the 
rain  and  must  be  renewed  several  times  during  the 
season.  It  is  odorless  and  clean  and,  when  the  atmosphere 
is  sufficiently  humid  to  supply  the  necessary  moisture, 
is  a  good  dust  layer.  On  residence  streets  or  suburban 
roads,  where  water  is  available,  it  may  be  found  a  satis- 
factory means  of  preventing  dust,  although  the  cost 
may  be  somewhat  greater  than  some  of  the  other  methods 
used  for  that  purpose.  In  several  instances  it  has  been 
used  at  a  cost  of  about  2  cents  to  4  cents  per  square  yard 
of  road  surface  per  annum,  with  the  calcium  chloride 
about  $15  per  ton. 


164      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

*  In  regard  to  ascertaining  and  regulating  the  strength 
of  the  solution,  the  most  convenient  method  is  to  deter- 
mine  its   specific   gravity   by   means   of   a   hydrometer. 
Accurate  determination  have  been  made  of  the  specific 
gravity  of  solutions  of  known  percentage  compositions, 
and,  as  hydrometers  graduated  to  direct    specific  gravity 
readings  can  be  obtained,  the  method  is  a  very  simple 
one.     A  hydrometer  graduated   from    i    to    1.4  is  most 
suitable  for  ordinary  work,  and  by  comparing  the  readings 
with  the  following  table,  the  strength  of  solution  at  15 
C.   can  be   immediately   ascertained.     Also   by   diluting 
the  salt  or  concentrated  solution  with  water,  any  desired 
strength  may  be  obtained  if  the  dilution  is  stopped  at 
the  specific  gravity  indicated  for  that  particular  strength. 

Per  cent  calcium  chloride     5          8          10        15         20        30        40 
Specific  gravity 1.041  1.068  1.086  1.132  1.182  1.286  1.402 

Sea-water.  Attempts  have  frequently  been  made 
to  use  sea-water  for  dust  prevention,  with  the  object  of 
reducing  the  number  of  sprinklings  necessary,  and  the 
cost  of  laying  the  dust,  through  taking  advantage  of  the 
presence  of  certain  hygroscopic  and  deliquescent  salts 
in  the  sea-water.  The  reliance  is  mainly  upon  magnesium 
chloride  (MgCl2)  which  is  always  found 'in  the  sea-water, 
and  which  possesses  the  desired  properties  to  a  somewhat 
less  degree  than  calcium  chloride.  It  has  been  found 
that  the  effect  of  sprinkling  is  more  lasting  than  when 
fresh  water  is  used  and  the  number  of  sprinklings  may 
be  lessened,  but  the  presence  in  the  water  of  other  salts 
not  possessing  hygroscopic  properties,  and  which  cause 
disagreeable  mud  in  wet  weather,  have  rendered  this 
treatment,  in  some  instances,  rather  unsatisfactory. 
It  has  been  claimed  that  this  salt  mud  is  injurious  to 

*  Hubbard,  Dust  Preventives  and  Road  Binders,  New  York,  1910. 


BROKEN-STONE   ROADS.  165 

horses'  feet,  and  destructive  to  the  iron  work  of 
vehicles. 

Oil.  Bituminous  materials  have  recently  come  into 
extensive  use  as  substitutes  for  water  in  laying  the  dust 
upon  macadam  roads.  These  may  be  used  as  tem- 
porary dust  preventives  or  as  permanent  road  binders. 
These  materials  will  be  discussed  in  detail  in  Chapter 
VI.  The  asphalts  and  tar  products  are  not  usually 
employed  as  temporary  dust  preventives,  but  the  petro- 
leums are  frequently  used  for  the  purpose.  Many  dif- 
ferent grades  of  oil  have  been  tried  with  varying  degrees 
of  success. 

Crude  Petroleums  are  very  commonly  employed.  Those 
having  an  asphaltic  base,  like  the  California  oils,  give  the 
best  results.  The  heavier  parts  of  these  oils  have  bind- 
ing properties  which  exert  a  lasting  effect  upon  the  road, 
when  the  more  volatile  portions  have  disappeared.  The 
petroleums  with  paraffine  bases,  however,  like  the  oils 
from  the  Pennsylvania  district, possess  no  binding  properties 
and  sometimes  produce  an  objectionable  slime  which 
makes  them  worse  than  useless.  The  semi-asphaltic 
oils  from  the  Texas  and  Kansas  fields  have  also  been 
fairly  successful,  but  residual  oils  obtained  from  these 
mid-continent  petroleums,  after  the  removal  of  the 
lighter  parts,  have  been  found  much  more  satisfactory, 
and  seem  to  be  the  best  material  available  for  ordinary 
sprinkling  in  the  Eastern  States,  where  the  cost  of  trans- 
portation would  prevent  the  use  of  California  petroleum. 

The  oil  is  easily  distributed  upon  the  surface  of  the  road 
by  means  of  the  ordinary  sprinkling  wagon,  J  to  ^  gallon 
being  required  per  square  yard  of  surface.  One  or  two 
applications  may  be  needed  each  season,  depending 
upon  the  character  of  traffic  upon  the  road,  and  the 
amount  and  character  of  oil  applied.  With  oil  costing 


1 66      A  TEXT-BOOK   ON  ROADS   AND   PAVEMENTS. 

4  to  5  cents  per  gallon,  the  cost  of  treatment  may  vary 
from  about  }  to  ij  cents  per  square  yard  of  road  surface. 
This  will  usually  be  considerably  cheaper  than  sprinkling 
with  water  or  the  use  of  calcium  chloride.  Oil  should 
not  be  applied  to  a  surface  covered  with  dust,  but  the 
road  should  be  clean  and  dry  when  the  oil  is  applied, 
and  dry  weather  immediately  following  the  application 
is  desirable  also,  in  order  to  lessen  the  somewhat  objec- 
tionable period  during  which  it  is  being  absorbed  into 
the  surface. 

Oil  Emulsions.  Various  methods  have  been  devised  for 
using  oils  in  the  shape  of  emulsions,  with  a  view  to  reduc- 
ing the  difficulties  and  costs  of  applying  the  oil  to  the 
surface  of  the  road.  There  are  quite  a  number  of  processes 
under  various  names,  some  of  them  patented,  and  most 
of  them  depending  upon  the  use  of  ammonia  or  soap 
solutions  as  emulsifiers.  A  machine  has  also  been 
invented  for  the  purpose  of  forming  emulsions  mechan- 
ically, without  the  use  of  saponifying  materials.  This 
is  known  as  the  "  Emuhifix,"  and  consists  of  a  wagon 
carrying  two  tanks,  one  for  the  oil  and  the  other  for 
water.  These  are  connected  by  pipes  with  a  third 
tank,  in  which  the  mixture  is  formed  by  rapidly  revolving 
blades,  which  also  force  the  mixture  upon  the  road  sur- 
face in  a  line  spray.  The  water  soon  disappears  leaving 
the  oil  in  a  finely  divided  state  over  the  surface  to  act  as 
a  dust  layer. 

The  effect  of  oil  emulsion  is  temporary  and  they  need 
to  be  applied  several  times  during  the  season.  The 
expense  may  be  somewhat  greater  than  where  a  single 
application  of  oil  is  made,  but  the  road  may  be  used 
immediately  following  the  application,  without  the 
objectionable  conditions  which  follow  the  use  of  a  larger 
quantity  of  oil.  Heavier  oils  may  be  used  as  emulsions 


BROKEN-STONE  ROADS.  167 

than  could  be  sprinkled  cold  without  the  water,  and  these 
may  act  as  binders  in  the  road  surface  after  a  number 
of  applications,  with  more  lasting  effect  than  the  lighter 
oils. 

Chemical  emulsions  are  on  the  market  in  concentrated 
form.  These  are  prepared  so  as  to  be  readily  miscible 
with  water  at  the  site  of  the  work  before  using.  When 
these  contain  a  good  oil  of  asphaltic  base  they  frequently 
give  good  service. 


CHAPTER  VI. 
BITUMINOUS  MACADAM  ROADS. 

ART.  43.    TYPES  OF  BITUMINOUS  MACADAM. 

THE  use  of  oil  upon  macadam  roads  as  a  means  of 
dust  laying  has  already  been  considered  in  Art.  42. 
Such  use  has  for  its  object  the  preservation  of  the  road 
through  preventing  the  formation  of  dust.  This  effect 
is  temporary  and  needs  to  be  frequently  repeated,  leaving 
no  permanent  binder  in  the  road  surface.  In  the  con- 
struction of  bituminous  macadam,  however,  the  purpose 
is  to  introduce  bitumen  into  the  road  surface  as  a  binder, 
in  order  to  increase  resistance  to  the  wear  of  traffic,  by 
cementing  the  surface  metal  firmly  together,  and  im- 
pregnating it  with  bitumen. 

The  methods  employed  in  constructing  bituminous 
macadam  roads  are  of  several  different  types: 

(a)  Surface  Treatment,  which  consists  in  applying 
a  coating  of  bitumen  to  a  finished  surface  of  water-bound 
macadam.  This  coating  is  applied  either  cold  or  heated, 
and  is  usually  covered  with  a  layer  of  sand  or  other  fine 
material,  and  rolled.  Two  applications  are  commonly 
made,  the  object  being  to  secure  an  even  coating  over 
the  road,  and  to  permit  the  bitumen  to  be  absorbed  into 
the  road  material  to  as  great  an  extent-as  possible. 

This  treatment  is  for  the  most  part  limited  to  the  treat- 
ment of  roads  already  built,  although  sometimes  employed 
in  new  construction.  For  old  roads  which  are  in  good 
surface,  it  forms  a  convenient  method  of  treatment, 

168 


BITUMINOUS  MACADAM  ROADS.  169 

without  renewing  the  surface.     The  details  of  the  method 
are  discussed  in  Art.  49. 

(b)  Penetration  Method.     In  constructing  a  bituminous 
macadam  surface  by  this  method,  the  macadam  is  first 
placed  in  the  ordinary  manner,  but  without  applying  the 
binder  to  the  surface  layer  of  stone.     A  coating  of  bitumen 
is  then  given  to  the  surface,  and  allowed  to  flow  into 
the  voids  in  the  stone,  after  which  stone  chips  are  applied 
and  rolled  into  the  surface.     A  second  coating  of  bitumen 
is  usually  applied  and  covered  with  additional  chips,  or 
screenings,   and  the  whole  rolled  to  a  smooth  surface. 

This  method  is  extensively  employed  in  the  United 
States,  the  materials  used  and  the  details  of  construction 
varying  widely  in  different  parts  of  the  country.  These 
are  discussed  in  Art.  50. 

(c)  Mixing  Method.     This  consists  of  using  for  the 
surface  layer,  bituminous  concrete,  obtained  by  mixing 
hot  bituminous  binder  with  the  macadam  stone.     The 
lower  course  of  the  road  is  usually  formed  of  water-bound 
macadam,  the  surface  material,  which  has  been  previously 
mixed,  is  then  placed  to  the  proper  thickness  and  rolled 
to   a  smooth   surface.     Sometimes   the   stone   is  heated 
before  mixing  with   the  bitumen,   and  sometimes   used 
cold.     Commonly  a  layer  of  stone  chips  is  rolled  into 
the  surface  of  the  concrete,  and  frequently  a  paint  coat 
of  bitumen  is  also  applied,  covered  with  sand  or  stone 
chips,  and  rolled  to  a  finished  surface. 

This  method  is  largely  used  in  England  and  to  a  less 
extent  in  the  United  States.  It  is  applied  mainly  to 
new  construction,  although  sometimes  used  in  resurfac- 
ing old  roads.  The  methods  used  in  construction  are 
discussed  in  Art.  51. 

In  the  construction  of  city  streets,  bituminous  con- 
crete is  frequently  used,  which  is  obtained  by  more  care.- 


170     A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

ful  and  elaborate  methods,  closely  graded  aggregates 
being  employed,  sometimes  with  cement  concrete  founda- 
tion. This  more  expensive  construction  is  not  classed 
under  the  head  of  bituminous  macadam,  but  is  commonly 
known  as  bitulithic  pavement  and  is  discussed  in  Chap- 
ter IX. 

(d)  Gladwell    Method.     This    method    was    developed 
in  England  as  a  means  of  resurfacing  old  roads,  but  is 
also  used  in  new  construction.     It  has  not  been  used 
to  any  extent  in  the  United  States. 

A  foundation  coarse  of  water-bound  macadam  is 
first  formed,  or  the  surface  of  the  existing  macadam  is 
smoothed  and  cleaned.  A  light  layer  (about  f  inch) 
of  tarred  chippings  is  then  spread,  and  upon  this  the 
road  metal  is  placed  and  rolled  into  the  chippings.  On 
top  of  the  macadam  surface  so  formed,  a  second  layer 
of  the  tarred  chippings  is  spread  and  rolled  into  the  voids 
in  the  surface  stone,  with  the  object  of  thoroughly  filling 
the  macadam  stone  with  the  matrix.  The  surface  is 
then  sealed  by  a  light  coating  of  hot  tar  preparation, 
which  is  covered  with  screenings  and  rolled. 

According  to  the  specifications  of  the  inventors  of  the 
method,  the  matrix  is  composed  of  clean  dry  granite 
screenings  mixed  warm  with  a  special  tar  preparation, 
while  the  aggregate  is  broken  to  nearly  uniform  size 
(2  ins.  to  2j  ins.).  The  good  results  which  have  been  ob- 
tained with  these  roads  in  England,  have  been  attributed* 
to  the  excellence  of  the  materials  used  in  forming  the 
matrix  rather  than  to  the  method  of  construction. 

(e)  Rock    AspJwlt    Macadam.     The    method    of    con- 
structing these  roads  is  to  form  the  macadam  surface 
in  the  same  manner  as  for  water-bound  macadam,  and 
then,  in  place  of  the  usual  binding  material,  to  apply 

*  Smith,  Dustless  Roads,  Tar  Macadam,  London,  1909. 


BITUMINOUS  MACADAM   ROADS.  171 

a  top  dressing  of  ground  rock  asphalt  and  roll  to  a  smooth 
surface.  The  surface  layer  of  stone  is  thoroughly  rolled, 
without  binder,  to  a  thickness  of  about  2\  or  3  inches, 
this  is  then  covered  with  a  thin  layer,  about  \  inch,  of 
the  rock  asphalt,  which  is  rolled  .thoroughly  into  the 
voids  in  the  stone.  A  thicker  layer,  about  i  inch,  of  the 
rock  asphalt  is  then  placed  and  rolled  to  a  smooth  sur- 
face. The  stone  for  the  surface  layer  is  usually  of  rather 
uniform  size,  i  inch  to  2  inches,  and  must  be  quite  dry 
when  used.  A  thin  cushion  of  the  rock  asphalt  should 
be  left  over  the  surface  upon  completion,  to  be  forced 
into  the  voids  in  the  surface  metal  under  the  action  of 
traffic,  although  it  is  desirable  that  the  wear  of  traffic 
come  upon  the  stone  of  the  surface  course  and  the  asphalt 
serve  purely  as  binder. 

When  the  asphalt  is  convenient  to  the  work  and  the 
cost  of  transportation  is  not  too  great,  these  roads  may 
often  be  economically  constructed  and  give  good  service. 
Rock  asphalt  for  this  purpose  should  contain  about 
7  to  10  per  cent  of  bitumen.  Kentucky  asphalt  has 
been  used  to  considerable  extent  for  this  purpose  very 
successfully. 

ART.  44.     BITUMINOUS  MATERIALS. 

The  term  bitumen  is  used  to  designate  a  class  of  sub- 
stances, consisting  of  a  mixture  of  various  series  of  hydro- 
carbons and  possessing  certain  physical  and  chemical 
properties  by  which  they  are  denned.  Several  groups 
of  hydrocarbons  are  commonly  present  in  each  sample 
of  bitumen,  and  the  range  of  possible  mixtures  is  very 
great,  so  that  considerable  confusion  exists  concerning  the 

assincation  of  the  various  substances  included  under 
this  designation,  as  well  as  in  denning  the  limits  within 
which  substances  may  be  considered  to  be  bitumens. 


172      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Mr.  Prevost  Hubbard  has  made  a  very  careful  study  of 
these  materials,  and  gives  the  following  definitions :  * 
"  Bitumens  may  be  described  as  consisting  of  a  mixture 
of  native  or  pyrogenetic  hydrocarbons  and  their  derivatives, 
which  may  be  gaseous,  liquid  or  solid,  but  if  solid,  melt- 
ing more  or  less  readily  upon  the  application  of  heat, 
and  soluble  in  chloroform,  carbon  bisulphide  and  similar 
solvents." 

Bitumens  being  thus  a  mixture  of  various  hydrocarbons 
in  differing  proportions,  have  no  fixed  compositions, 
and  vary  widely  in  properties  according  to  the  characters 
and  amounts  of  the  hydrocarbons  of  which  they  are 
composed.  They  are  divided  into  natural  bitumens 
and  artificial  bitumens.  Natural  bitumens  are  those 
which  occur  in  nature  as  mineral  bitumens.  Artificial 
bitumens  are  those  which  are  formed  by  the  distillation  of 
certain  other  mineral  substances  known  as  pyrobitumens. 
The  distillation  of  natural  bitumens  also  gives  rise  to 
other  bitumens  of  differing  characteristics,  and  these, 
particularly  those  left  as  residues  after  distillation,  are  also 
designated  artificial  bitumens. 

NATIVE  BITUMENS. 

Bitumens  occur  in  nature  as  natural  gases,  petroleums, 
malthas  and  asphalts,  beside  a  number  of  other  materials 
which  gradually  merge  into  the  pyrobitumens. 

Petroleums  are  liquid  bitumens  and  are  divided  into 
parafrme  oils,  cylic  or  asphaltic  oils  and  semi-asphaltic 
oils.  The  parafifrne  petroleums  consist  mainly  of  the 
paraflfine  hydrocarbons  (CnH2n  +  2)  and  are  of  little 
importance  as  road  materials,  showing  no  permanence 
as  dust  preventives  and  no  binding  properties  in  the  road. 
The  asphaltic  petroleums  are  characterized  by  the  poly- 

*  Dust  Preventives  and  Road  Binders,  New  York,  1910. 


BITUMINOUS  MACADAM  ROADS.  173 

methylene  hydrocarbons  and  when  distilled  yield  a  residue 
similar  in  character  to  asphalt.  The  semi-asphatic  oils 
consist  of  a  mixture  of  the  paraffine  hydrocarbons  with 
those  of  the  asphaltic  oils. 

Asphalts  are  solid  bitumens  composed  of  hydrocarbons 
of  the  same  characters  as  those  of  the  asphaltic  oils,  but 
with  the  lighter  and  more  volatile  parts  removed.  This 
class  includes  the  true  asphalts  and  a  number  of  similar 
materials,  such  as  glance  pitch,  manjak  and  gihonite. 

Malthas  are  heavy  oils  intermediate  between  the  asphal- 
tic petroleums  and  the  asphalts.  They  are  similar  in 
character  to  the  fluid  residuums  derived  from  asphaltic 
petroleums,  but,  as  they  contain  more  of  the  volatile 
hydrocarbons,  may  be  hardened  by  exposure  to  the  air, 
or  by  heat,  becoming  an  artificial  asphalt.  These  mate- 
rials have  been  produced  to  some  extent  in  California  and 
seem  to  form  desirable  road  materials. 

ARTIFICIAL  BITUMENS. 

The  artificial  bitumens  which  are  of  importance  as 
road  materials  are  crude  tars,  residues  from  the  dis- 
tillation of  petroleums  and  residues  from  the  distillation 
of  tars.  The  crude  tars  include  water-gas  tars,  and  coal 
tars.  Residues  from  the  distillation  of  petroleum  are 
either  residual  oils  or  residual  pitches.  The  residual 
oils  are  obtained  by  removing  all  of  die  more  volatile 
oils,  including  the  lubricating  oih.  These  may  be  used 
as  road  oils,  when  derived  from  asphaltic  petroleum, 
and  are  aho  employed  as  fluxing  agents  in  preparing 
asphalts  for  use  in  pavements.  Residual  pitches  are 
obtained  when  distillation  is  carried  far  enough  to  leave 
a  solid  residue.  If  derived  from  asphaltic  oil  these  may 
give  good  road  materials. 


174      A   TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

The  residues  derived  from  the  distillation  of  tars  are 
known  as  dehydrated  tars,  refined  tars  and  tar  pitches. 
Dehydrated  tar  is  that  from  which  the  water  has  been 
removed.  Tar  pitches  are  formed  when  distillation 
leaves  solid  or  semi-solid  residues.  Refined  tars  are 
products  intermediate  between  dehydrated  tars  and 
pitches.  All  of  these  materials  may  be  of  use  as  road 
binders  when  carefully  prepared  from  coal-tar. 

ART.  45.     PETROLEUMS. 

Petroleums,  or  mineral  oils,  are  widely  distributed 
throughout  the  earth.  They  occur  in  generous  quantities 
in  many  parts  of  the  United  States,  the  oils  of  different 
localities  varying  widely  in  character. 

The  Applachian  oil  field  lies  on  the  west  side  of  the 
Alleghany  Mountains,  through  Western  Pennsylvania, 
West  Virginia,  Kentucky  and  Tennessee.  These  oils, 
particularly  those  known  as  Pennsylvania  grade,  are 
rich  in  paraffines,  and  contain  practically  no  asphaltic 
hydrocarbons.  They  are  commercially  the  most  valuable 
of  the  petroleums,  on  account  of  the  large  amount  of 
illuminating  oils  derived  from  them,  but  are  of  little 
value  for  road  purposes,  as  the  base  retained  after  evapora- 
tion of  the  more  volatile  constituents  is  lacking  in  binding 
properties. 

The  Ohio-Indiana  oilfield  includes  the  oils  of  Western 
Ohio  and  Indiana.  They,  like- the  Pennsylvania  oils,  are 
of  little  value  for  road  purposes,  but  they  differ  from  the 
Pennsylvania  oils  in  containing  considerable  sulphur  and 
in  giving  a  less  amount  of  illuminating  oils. 

The  Illinois  oil  field  covers  the  oil  districts  in  Illinois 
and  a  part  of  Northern  Kentucky.  These  oils  vary 
considerably  in  character,  and  seem  to  be  of  less  com- 


BITUMINOUS  MACADAM  ROADS.  175 

mercial  value  than  the  oils  of  the  more  eastern  fields. 
Some  are  quite  similar  to  those  of  the  Ohio-Indiana 
field,  while  others  contain  considerable  percentages  of 
the  asphaltic  hydrocarbons  and  approach  the  semi- 
asphaltic  oils  in  character.  The  recent  development 
of  this  field  has  been  very  rapid,  and  large  quantities 
of  these  oils  are  now  being  produced. 

The  mid-continent  oil  field  comprises  the  Kansas  and 
Oklahoma  oils  and  those  of  Northern  Texas.  These 
oils,  like  those  of  Illinois,  are  quite  varied  in  character. 
They  contain  considerable  quantities  of  the  asphaltic 
hydrocarbons  as  well  as  those  of  the  paraifine  series 
and  may  be  classed  as  semi-asphaltic.  Very  large 
quantities  of  petroleum  .are  produced  in  this  district, 
which  are  sold  at  lower  prices  than  any  of  the  others. 

The  Gulf  oil  field  includes  Louisiana  and  Texas.  These 
oils  usually  contain  more  asphaltic  and  less  paraffine 
hydrocarbons  than  the  Illinois  or  Kansas  oils.  They 
yield  residues  superior  to  the  others  as  road  materials, 
because  of  possessing  better  binding  properties. 

The  California  oil  field  produces  oils  composed  mainly 
of  asphaltic  hydrocarbons.  These  oils  vary  widely 
in  density,  the  lighter  ones  being  less  suitable  for  road 
purposes,  unless  refined,  on  account  of  the  greater  per- 
centages of  volatile  hydrocarbons  present.  When  dis- 
tilled, they  yield  residues  similar  in  character  to  asphalt, 
which  have  been  employed  for  pavements  under  the 
name  of  artificial  asphalts. 

Crude  petroleums  are  frequently  used  as  dust  layers 
on  roads,  and  all  classes  of  oils  have  been  tried  for  this 
purpose*  The  paraffine  oils  are  of  use  for  very  temporary 
effect  only.  These  oils  are  also  objectionable  on  account 
of  the  sticky  black  mud  frequently  formed  in  wet  weather. 
The  semi-asphaltic  oils  of  the  mid-continent  and  Texas 


1 76     A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

fields  are  more  satisfactory,  not  possessing  the  objectionable 
properties  of  the  paraffine  oils,  but  the  effect  of  these 
when  crude  oil  is  used  is  also  lacking  in  permanence  and 
does  not  exert  a  binding  effect  upon  the  road  surface. 
The  asphaltic  petroleums  of  the  California  field  have 
frequently  given  good  results  when  used  in  a  crude  state, 
binding  together  the  material  of  the  road  surface,  and 
causing  it  to  gradually  harden.  The  large  quantity 
of  excellent  material  available  for  such  use  at  low  cost 
has  led  to  the  development  of  the  oiled  earth  road  in 
California  (discussed  in  Art.  29)  as  well  as  to  the  extensive 
use  of  oiled  macadam  roads. 

Petroleum  Residuums.  The  character  of  the  residue 
obtained  by  the  distillation  of  petroleum  depends  upon 
the  kind  of  petroleum  used  and  the  extent  to  which  the 
distillation  is  carried.  The  residuums  may  be  either 
fluid  or  solid,  according  to  the  extent  to  which  the  lighter 
oils  have  been  driven  off. 

The  fluid  residues  from  the  semi-asphaltic  petroleums 
are  largely  used  as  road  oils,  under  the  name  of  asplialt 
oils.  Many  of  these  possess  good  binding  properties, 
and  are  satisfactory  materials  for  the  purpose.  These 
oils  are  also  frequently  employed  as  fluxes  for  the  solid 
bitumens  in  preparing  paving  mixtures. 

The  binding  properties  of  road  oil  depend  upon  the 
presence  of  the  heavy  asphaltic  hydrocarbons,  which 
possess  adhesive  properties  and  will  remain  in  the  road 
after  the  evaporation  of  the  lighter  hydrocarbons.  Oils 
for  this  purpose  therefore  should  contain  a  considerable 
percentage  of  the  heavy  hydrocarbons  (bitumen  insoluble 
in  88  degree  naphtha)  and  should  have  a  low  percentage 
of  paraffine  scale. 

The  solid  residuums  from  asphaltic  or  semi-asphaltic 
petroleum  are  commonly  known  as  oil  asphalts,  and  are 


BITUMINOUS  MACADAM  ROADS.  177 

employed  in  much  the  same  manner  as  the  natural 
asphalts.  These  materials  vary  greatly  in  character, 
depending  upon  the  care  used  in  preparation  as  well  as 
the  petroleum  from  which  they  are  made. 

Blown  oils  are  semi-solid  residuums  prepared  by  blow- 
ing air  through  a  fluid  residuum,  causing  a  thickening 
of  the  oil.  The  oxygen  of  the  air  combines  with  a  part 
of  the  hydrogen  of  the  hydrocarbons,  thus  producing  a 
change  in  the  characters  of  the  hydrocarbons. 

These  oils  have  been  used  to  considerable  extent  for 
road  purposes  when  made  from  semi-asphaltic  petroleums, 
and,  when  carefully  prspared,  have  shown  good  results. 


ART.  46.    SOLID  NATIVE  BITUMENS. 

The  solid  native  bitumens  which  are  of  interest  in 
paving  or  road  work  include  the  asphalts,  gilsonites  and 
grahamites.  These  bitumens  consist,  like  the  petroleums, 
of  natural  mixtures  of  hydrocarbons,  but  are  composed 
mainly  of  the  heavier  hydrocarbons  and  occur  as  solids. 
Mr.  Clifford  Richardson  divides*  the  hydrocarbons 
occurring  in  thess  bitumens  into  foar  classes  which  he 
calls: 

Petrolenes,  including  those  hydrocarbons  which  are 
volatilized  at  325  F.  in  7  hours. 

Malthenes,  including  the  oils  which  are  soluble  in  88° 
Baume  naphtha. 

Asphaltenes,  including  the  heavier  hydrocarbons  not 
soluble  in  naphtha  but  soluble  in  cold  carbon  tetrachloride. 

Carbenes,  including  hydrocarbons  soluble  in  carbon 
bisulphide,  but  insoluble  in  cold  carbon  tetrachloride. 

This  classification  is  also  frequently  extended  to  the 

*  The  Modem  Asphalt  Pavement,  New  York,  1905. 


178      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

hydrocarbons  occurring  in  the  petroleums,  which  are 
very  similar  in  character. 

The  solid  native  bitumens  can  only  be  used  for  road 
purposes  by  being  combined  with  fluid  bitumens  to 
bring  them  to  proper  consistency.  In  the  use  of  these 
materials  as  road  binders,  petroleum  residuum  is  employed 
as  a  flux  to  form  asphaltic  cement  in  the  same  manner 
as  in  the  construction  of  asphalt  pavement  (see  Art.  67). 

The  asphalts  consist  mainly  of  malthenes  and 
asphaltenes,  usually  mixed  with  a  certain  amount  of 
mineral  matter,  and  need  to  be  refined  before  using  to 
remove  water  and  impurities  thsy  may  contain.  They 
are  produced  to  only  a  limited  extent  in  the  United  States, 
and  while  used  largely  in  the  construction  of  asphalt 
pavements,  are  employed  very  little  for  bituminous 
macadam  roads  on  account  of  the  greater  cost  as  com- 
pared with  petroleum  and  tar  products. 

Gilsonite  differs  from  asphalt  in  being  more  largely 
composed  of  unsaturated  hydrocarbons,  and  less  soluble 
in  naphtha.  It  occurs  in  Colorado  and  Utah,  and  is  very 
nearly  pure  bitumen,  containing  very  little  mineral 
matter.  It  is  used  to  some  extent  in  the  construction 
of  asphalt  pavements,  as  well  as  in  macadam  road  con- 
struction for  which,  when  properly  prepared,  it  seems  a 
desirable  material. 

Grahamite  differs  from  asphalt  in  being  composed 
mainly  of  asphaltenes  and  carbenes,  as  it  is  but  slightly 
soluble  in  naphtha,  and  much  less  soluble  in  carbon 
tetrachloride,  than  the  other  asphaltic  materials.  It  has 
been  used  in  asphalt  pavements. 

These  materials  and  the  methods  of  using  them  are 
discussed  more  fully  in  Chapter  IX. 

Rock  Asphalt  is  the  name  applied  to  sandstone  or 
limestone  impregnated  with  bitumen.  These  materials 


BITUMINOUS   MACADAM   ROADS.  179 

differ  widely  in  the  character  of  the  rock,  as  well  as  in 
the  kind  and  quality  of  the  bitumen  with  which  it  is 
impregnated.  It  may  vary  from  sand,  in  which  the 
individual  grains  are  held  together  only  by  the  bitumen, 
to  solid  rock  in  which  the  pores  are  filled  with  bitumen. 
Some  of  these  materials  are  largely  used  for  asphalt 
pavements  (see  Chapter  IX).  They  are  also  used  to 
some  extent  as  fillers  in  bituminous  macadam  construc- 
tion. Materials  of  this  character  are  somewhat  widely 
distributed  over  the  United  States,  but  on  account  of 
the  wide  variation  in  the  material,  much  of  it  is  not 
suitable  to  this  use.  Good  results  have  been  obtained 
in  the  use  of  Kentucky  rock  asphalt,  consisting  of  sand- 
stone impregnated  with  about  6  to  8  per  cent  of  rather 
soft  bitumen,  which  hardens  upon  exposure  to  the  air, 
through  the  volatilization  of  some  of  the  light  oils. 

ART.  47.    TAR  PRODUCTS. 

The  coal  tar  used  in  road  work  is  obtained  as  a  by- 
product either  in  the  manufacture  of  illuminating  gas 
or  in  the  burning  of  coke.  Very  large  quantities  of  tar 
are  produced  in  the  United  States,  although  until  quite 
recently,  the  value  of  tar  has  been  so  small  that  it  has 
been  considered  a  necessary  evil,  and  but  little  attention 
given  to  the  character  of  the  tar  produced. 

Tars  are  made  up  of  a  mixture  of  hydrocarbons  of 
extremely  variable  character,  and  it  is  customary  to 
classify  these  as  light  oils  (volatilizing  below  170°  C.), 
heavy  oils  (volatilizing  between  170°  and  270°  C.),  and 
pitch,  which  is  the  residue  not  volatilized.  The  light 
oils  are  also  frequently  divided  into  those  which  volatilize 
below  110°  C.  and  those  volatilizing  between  110°  C. 
and  170°  C.  Crude  tars  commonly  contain  some 


180    A     TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

ammoniacal  water,  which  is  included  in  the  distillate  at 
110°  C.,  and  is  objectionable  in  tars  for  road  work,  if 
present  in  appreciable  quantities. 

The  value  of  tar  for  road  purposes  depends  upon  the 
method  and  care  used  in  its  production,  as  well  as  upon 
the  character  of  the  coal  from  which  it  is  obtained.  The 
temperature  at  which  the  coal  is  distilled  has  an  important 
effect  upon  the  quality  of  the  tar,  as  the  oils  formed  at 
different  temperatures  differ  considerably  in  character. 

In  the  manufacture  of  coal  gas,  bituminous  coal  is 
heated  in  a  retort  until  the  more  volatile  parts  are  driven 
off,  leaving  a  residue  of  coke.  The  gas  and  tar  are  sep- 
arated by  passing  the  distillate  through  water  under 
which  the  tar  is  condensed,  the  gas  passing  on  through 
a  condenser  and  other  apparatus  in  which  the  remainder 
of  the  tar  is  removed.  The  chemical  changes  which 
occur  in  the  distillation  of  coal  are  but  little  understood. 
They  consist  in  a  breaking  up  of  the  compounds  of  which 
the  coal  is  composed  and  the  formation  of  new  combina- 
tions, which  depend  upon  the  temperature  at  which  the 
distillation  takes  place.  At  a  high  temperature  the 
dissolution  is  more  complete,  a  greater  quantity  of  gas 
being  formed,  leaving  less  of  the  heavy  oils  in  the  tars, 
which  contain  more  solid  bitumens  and  more  free  carbon 
than  those  formed  at  lower  temperatures.  The  tars 
formed  at  high  temperatures  are  not  so  desirable  for  road 
purposes  on  account  of  the  lack  of  a  sufficient  quantity 
of  the  heavy  oils,  and  because  of  an  excess  of  free  carbon. 

Coke-oven  tar  is  obtained  from  coal  in  much  the  same 
manner  as  that  produced  in  the  manufacture  of  gas, 
excepting  that  the  chief  object  is  to  produce  coke  instead 
of  gas.  Large  quantities  of  tar  are  now  obtained  from 
this  source,  but  only  a  comparatively  small  portion  of 
the  coke  ovens  in  use  are  constructed  with  a  view  to 


BITUMINOUS  MACADAM  ROADS.  l8l 

saving  the  by-products,  and  in  most  instances  the  gas 
and  tar  are  allowed  to  escape  and  no  effort  made  to  save 
them.  The  great  increase  in  the  uses  of  tar  and  tar 
products  is,  however,  causing  a  change  in  this  respect, 
and  much  greater  quantities  of  tar  may  reasonably  be 
expected  in  the  future  from  this  source.  These  tars  are 
similar  to  those  from  gas  works,  but  are  usually  formed 
at  lower  temperatures,  and  thus  contain  larger  per- 
centages of  heavy  oils  and  less  free  carbon. 

Water-gas  tars  are  produced  by  the  decomposition 
of  petroleums,  or  petroleum  distillates,  in  the  carburetting 
of  water  gas.  The  petroleum  oils  are  broken  up  into 
light  oils,  or  gases,  which  impart  illuminating  value 
to  the  water  gas,  and  heavier  oils,  which  are  condensed 
as  tar.  These  tars  are  lighter  materials  than  the  coal 
tars,  containing  a  larger  percentage  of  heavy  oils  and 
less  of  pitch  residue.  They  are  usually  low  in  free  car- 
bon, do  not  contain  ammonia  water,  and  frequently 
are  desirable  materials  for  use  as  dust  layers  in  road  work. 

REFINED  TARS 

Tars,  like  petroleums,  are  refined  by  fractional  dis- 
tillation, the  character  of  the  residual  depending  upon 
the  extent  to  which  the  lighter  oils  have  been  driven 
off,  as  well  as  upon  the  nature  of  the  original  tar.  In 
refining,  the  tar  is  divided  into  several  fractions  by  sep- 
arating the  distillates  between  certain  temperatures,  and 
these  fractions  are  again  distilled  to  separate  into  desired 
products,  such  as  benzol,  naphtha,  carbolic  acid  and 
naphthalene. 

The  residue  may  be  liquid  or  solid  according  to  the 
temperature  to  which  the  distillation  has  been  carried, 
and  the  extent  to  which  the  heavy  oils  have  been  removed. 


1 82      A  TEXT-BOOK  ON  ROADS    AND   PAVEMENTS. 

The  semi-solid  and  solid  residues  are  termed  pitches 
and  are  classified  according  to  the  temperature  required 
to  liquefy  them.  Sufficient  heavy  oil  must  be  left  in 
the  residue  to  render  it  semi-fluid  if  it  is  to  be  used  as  a 
road  binder.  The  solid  residues  are,  however,  some- 
times used  in  the  same  manner  as  the  solid  native  bitu- 
mens, by  fluxing  them  with  other  oils. 

In  some  instances,  when  the  distillation  is  carried  so 
far  as  to  remove  the  heavy  oils,  leaving  a  solid  residue, 
parts  of  the  heavy  oils  are  returned  to  the  residuum 
before  it  cools,  thus  reducing  it  to  a  semi-fluid  condition, 
and  forming  what  is  known  as  a  cut-back  product.  Usually, 
where  this  method  is  followed,  the  naphthalene  is  removed 
from  the  heavy  oil,  which  is  an  advantage  to  the  use  of 
the  residual  for  road  work. 

DEHYDRATED  TARS 

Sometimes  when  tars  are  not  to  be  distilled  for  the 
separation  of  the  products  obtained  from  the  oils,  they 
are  prepared  for  use  in  road  work  by  heating  sufficiently 
to  drive  off  the  water  and  some  of  the  light  oils  and  are 
then  known  as  dehydrated  tars.  These  tars  are  superior 
to  crude  tars,  as  the  presence  of  water  is  objectionable 
in  tars  for  use  as  road  binders.  This  is  particularly  the 
case  with  those  containing  ammoniacal  water. 

ART.  48.    TESTS  FOR  BITUMINOUS  MATERIALS. 

On  account  of  the  wide  variation  in  character  of 
bituminous  materials  which  may  be  available  for  road 
work,  it  is  very  essential  that  tests  be  applied  for  the 
purpose  of  determining  the  properties  of  the  various 
bitumens  and  their  suitability  for  use  in  construction. 


BITUMINOUS   MACADAM  ROADS.  183 

Considerable  variation  exists  in  the  tests  applied  for  this 
purpose  by  various  authorities,  as  well  as  in  the  methods 
of  conducting  them,  and  but  little  has  been  accomplished 
towards  standardizing  such  work.  The  tests  here  enu- 
merated are  all  used  to  some  extent  and  are  of  value  for 
some  materials,  but  all  of  them  are  not  applicable  to  any 
one  material. 

SPECIFIC  GRAVITY. 

The  determination  of  specific  gravity  is  nearly  always 
important.  The  method  of  determination  must  of  coarse 
depend  upon  the  consistency  of  the  bitumen.  As  the 
specific  gravity  of  bitumens  varies  with  the  temperature, 
it  is  essential  that  the  determination  be  made  at  standard 
temperature,  and  25°  C.  (77°  F.)  is  ordinarily  employed 
for  the  purpose.  It  is  customary  to  state  the  specific 
gravity  in  terms  of  that  of  water  at  the  same  temperature. 

When  the  bitumen  is  quite  liquid,  a  hydrometer  gradu- 
ated to  read  the  specific  gravity  directly  is  the  most 
convenient  method  and  is  commonly  employed.  When 
the  materials  are  too  viscous  to  permit  the  use  of  a  hydrom- 
eter, it  is  usual  to  employ  a  picnometer.  Mr.  Hubbard 
describes  *  a  form  of  picnometer  specially  suited  to  this 
work,  while  Sommer  has  devisedf  a  specific  gravity 
apparatus  intended  for  semi-solid  and  solid  bitumens 
by  suspending  a  cup  containing  a  definite  volume  of  the 
bitumen  from  a  hydrometer,  the  stem  of  which  is  grad- 
uated to  read  the  specific  gravity. 

A  knowledge  of  the  specific  gravity  of  a  bitumen  is 
useful  both  as  assisting  in  determining  the  character  of 
the  bitumen  and  in  indicating  the  treatment  to  which  it 
may  have  been  subjected.  Crude  paraffine  petroleums 

*  Dust  Preventives  and  Road  Binders,  New  York,  IQIO. 
f  Proceedings  American  Society  for  Testing  Materials,  1909. 


184      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

vary  in  specific  gravity  from  about  0.75  to  0.85,  and  is 
usually  lower  than  asphaltic  petroleum,  which  may  have 
a  specific  gravity  of  about  0.92  to  0.97,  the  semi-asphaltic 
oils  being  between  the  two.  Residual  oils  are  heavier 
than  crude  oils  of  the  same  character,  the  difference 
being  to  some  extent  indicative  of  the  extent  of  distilla- 
tion to  which  the  oil  has  been  subjected.  The  specific 
gravity  of  coal  tar  is  largely  influenced  by  the  amount  of 
free  carbon  it  contains.  Refined  tars  suitable  for  road 
purposes  usually  vary  in  specific  gravity  from  about 
1.15  to  1.22,  the  higher  value  representing  a  rather  large 
percentage  of  free  carbon. 

COMPOSITION  OF  BITUMINOUS  MATERIALS. 

In  all  examinations  of  bituminous  materials  it  is  nec- 
essary to  determine  the  percentages  of  bitumen  and  of 
other  organic  and  inorganic  matter  in  the  materials. 
For  petroleums  and  solid  native  bitumens  it  is  customary 
to  determine  the  quantity  of  organic  matter  not  classed 
as  bitumen,  and  the  percentage  of  inorganic  material 
present.  For  tars,  it  is  usually  necessary  to  determine 
also  the  percentage  of  free  carbon. 

In  testing  native  bitumens,  the  light  oils  are  separated 
from  tha  heavy  ones  by  testing  the  solubility  in  86°  Be. 
or  88°  Be.  naphtha,  and  with  asphalts  the  carbenes  are 
separated  by  the  solubility  in  cold  carbon  tetrachloride. 
In  the  examination  of  tars  the  light  and  heavy  oils  are 
separated  by  fractional  distillation. 

i  Total  Bitumen.  All  organic  matter  which  is  soluble 
in  carbon  bisulphide  is  classed  as  bitumen.  Methods  of 
making  the  test  as  adopted  by  the  American  Society  for 
Testing  Materials  are  given  in  Art.  68.  The  following 
more  rapid  method,  which  has  also  been  recommended 


BITUMINOUS  MACADAM  ROADS.  185 

by  a  committee  of  the  same  society,  is  more  commonly 
used  in  testing  road  materials. 

"  From  i  to  10  grams  of  the  water-free  material  (de- 
pending upon  the  amount  of  bitumen  present)  is  weighed 
into  a  150-c.c.  Erlerimeyer  flask,  the  tare  of  which  has 
been  previously  ascertained,  and  treated  with  100  c.c. 
of  carbon  disulphide.  The  flask  is  then  loosely  corked 
and  shaken  from  time  to  time  until  practically  all  large 
particles  of  the  material  have  been  broken  up,  when  it 
is  set  aside  for  not  less  than  15  hours.  At  the  end  of  this 
time  the  contents  of  the  flask  are  decanted  off  upon  a 
weighed  Gooch  crucible  fitted  with  long-fiber  amphibole 
asbestos  filter.  The  residue  remaining  in  the  flask  is 
then  washed  with  50  cic.  of  carbon  disulphide,  allowed 
to  settle,  and  decanted  as  before,  the  insoluble  matter 
being  finally  brought  upon  the  filter  and  washed  with 
100  c.c.  carbon  disulphide,  or  until  the  washings  are 
practically  colorless.  The  filter  and  contents  are  then 
dried  at  125°  C.,  cooled,  and  weighed.  Should  any 
residue  remain  in  the  flask,  it  is  also  dried  and  weighed, 
and  this  weight  added  to  that  of  the  residue  in  the 
crucible.  The  filtrate  should  be  burned  off  and  ignited 
to  an  ash,  and  the  weight  of  the  ash  thus  obtained  added 
to  that  of  the  insoluble  residue.  The  weight  of  the  total 
residue  deducted  from  that  of  the  original  material 
gives  the  weight  of  bitumen  soluble  in  cold  carbon 
disulphide.  In  case  of  tars  and  pitches  the  percentage 
of  insoluble  residue,  determined  as  above,  minus  that 
of  any  ash  which  may  be  found  by  igniting  a  separate 
sample,  is  reported  as  free  carbon." 

Free  Carbon.  In  the  examination  of  coal  tars,  or  tar 
products,  the  determination  of  free  carbon  is  a  matter 
of  importance,  as  this  is  usually  considered  an  undesirable 
constituent  in  a  road  tar,  and  limits  are  commonly  set 


1 86       A  TEXT-BOOK  ON  ROADS   AND   PAVEMENTS. 

in  specifications  to  the  percentage  which  may  be  present. 
A  method  of  determining  free  carbon  is  given  above 
in  the  test  for  total  bitumen. 

Fixed  Carbon.  The  term  fixed  carbon  is  applied  to 
the  residual  coke  resulting  from  burning  bitumen  in  a 
closed  crucible,  and  in  the  absence  of  oxygen.  The 
test  is  frequently  used  in  the  examination  of  petroleum 
residuums  and  asphalts  to  indicate  the  extent  to  which 
these  materials  contain  the  heavier  hydrocarbons  of 
asphaltic  character.  A  method  of  conducting  the  test 
as  recommended  by  a  Committee  of  the  American  Society 
of  Civil  Engineers  is  as  follows: 

"  About  i  gram  of  the  compound  is  weighed  into  a 
platinum  crucible  i|  to  ii-  inches  high.  The  crucible 
with  the  lid  on  is  heated,  first  gently,  and  then  until  no 
more  smoke  and  flame  issues  between  the  crucible  and 
the  lid.  It  is  then  heated  3^  minutes  in  the  full  heat  of 
the  burner;  then  cooled  and  weighed.  The  crucible 
lid  is  then  removed  and  the  crucible  and  contents  allowed 
to  remain  in  the  full  heat  of  the  burner  until  the  carbon 
is  burned  off,  and  then  weighed  again.  The  difference 
between  these  two  weights  is  the  fixed  carbon. 

Naphtha  Soluble  Bitumen.  Petroleum  and  asphalt 
bitumens  are  commonly  tested  as  to  their  solubility  in 
naphtha  derived  from  paraffine  petroleum.  This  test  is 
made  to  determine  the  relative  proportions  of  the  heavier 
hydrocarbons  (asphaltenes) ,  which  are  insoluble,  and  of 
the  oils  (malthenes),  which  are  soluble  in  the  naphtha. 

Naphtha  for  this  purpose  is  commonly  required  to  have 
a  density  of  88°  Be.  and  a  boiling-point  between  40°  C. 
and  55°  C.  Some  authorities  prefer  to  use  naphtha  of 
86°  Be.  gravity,  on  account  of  it  being  more  easily 
obtainable,  although  it  dissolves  a  little  more  of  the 
bituminous  material  and  is  not  quite  so  satisfactory  for 


BITUMINOUS  MACADAM  ROADS.  187 

this  reason.  In  reports  of  this  test,  the  density  of  the 
naphtha,  and  temperatures  between  which  it  distills  should 
always  be  given,  and  results  should  be  stated  in  terms  of 
total  bitumen.  This  test  is  conducted  in  the  same  manner 
as  that  for  solubility  in  carbon  disulphide. 

Distillation  Test.  This  test  is  made  upon  tars  to 
separate  the  oils  which  distill  at  different  temperatures, 
and  determine  the  proportions  of  each  contained  by  the 
tar. 

The  test  is  made  by  heating  the  tar  in  a  retort,  and 
collecting  the  distillate  in  a  condenser  pipe,  which  is 
changed  as  each  temperature  is  reached  in  the  distilla- 
tion, and  the  distillate  cooled  to  standard  temperature 
and  its  volume  measured  or  its  weight  taken. 

Mr.  Hubbard  recommends  *  the  use  of  temperatures 
110°  C.,  170°  C.,  and  270°  C,  as  points  of  division. 
The  fraction  which  distills  over  up  to  110°  C.,  includes 
the  water  and  ammonia  compounds,  with  certain  light 
oils.  These  separate  in  the  condenser  and  may  be 
separately  measured.  The  distillates  between  no  C. 
and  170°  C.,  are  regarded  as  light  oils;  those  between 
170  C.  and  270°  C  as  heavy  oils,  and  the  residue  as  pitch. 

In  tars  for  road  binders,  water  and  ammonia  compounds 
should  not  be  present  in  appreciable  quantities,  but  a 
small  percentage  of  light  oils  is  desirable,  and  if  the  tar 
is  to  be  used  as  a  binder  in  construction  of  bituminous 
macadam,  it  should  contain  at  least  50  per  cent  of  mate- 
rial not  volatilized  at  270°  C. 

TESTS  FOR  CONSISTENCY. 

For  the  purpose  of  determining  the  consistency  of 
bituminous  materials,  three  tests  are  commonly  employed; 

*  Dust  Preventives  and  Road  Binders,  New  York,  1910. 


1 88      A  TEST-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  viscosity  test  for  fluid  bitumens,  the  float  test  for 
materials  too  viscous  for  the  viscosity  test,  and  the  penetra- 
tion test  for  solid  materials. 

Viscosity  Test.  Viscosity  is  measured  by  determining 
the  time  required  for  a  given  volume  of  the  liquid  under 
test  to  pass  through  a  small  opening,  and  comparing 
with  the  time  required  for  the  same  volume  of  water, 
at  standard  temperature  (25°  C.),  to  pass  the  same 
opening. 

The  Engler  viscosimeter  is  commonly  employed  for 
this  purpose.  It  consists  of  a  covered  brass  vessel  with 
a  conical  bottom,  to  which  is  fitted  a  vertical  outflow 
tube  20  mm.  long,  with  a  diameter  at  top  of  2.9  mm. 
and  at  bottom  of  2.8  mm.  The  tube  is  closed  by  a  hard- 
wood stopper,  which  extends  through  the  cover  of  the 
vessel.  The  vessel  sets  in  a  bath  of  heavy  oil  which  is 
heated  by  a  ring  burner  underneath.  A  measuring 
flask  is  placed  beneath  the  outflow  tube  to  receive  the 
liquid  as  it  passes  through  the  opening.  The  time 
required  by  a  given  volume  of  the  bitumen  at  the  desired 
temperature  to  pass  the  opening  is  measured,  and  the 
viscosity  computed  by  dividing  the  time  so  obtained 
by  the  time  required  for  the  same  volume  of  water  at 
25°  C.  to  pass  through. 

Float  Test.  This  test  is  sometimes  applied  to  materials 
too  viscous  for  the  Engler  viscosimeter,  The  New  York 
Testing  Laboratory  Float  Apparatus  *  is  employed  in 
making  the  test.  It  consists  of  a  float  or  saucer,  with 
an  opening  in  the  bottom,  into  which  a  brass  collar  may 
be  screwed.  The  collar  is  filled  with  the  bitumen 
(softened  by  heat),  and  then  placed  in  ice  water  at  41°  F. 
for  15  minutes.  The  collar  is  then  screwed  into  the 
float,  which  is  placed  upon  the  surface  of  water  at  90°  F. 

*  See  Engineering  Record,  Vol.  LIX,  584. 


BITUMINOUS  MACADAM  ROADS.  189 

When  the  bitumen  softens,  water  is  admitted  to  the 
saucer  and  the  apparatus  sinks.  The  time  in  seconds  re- 
quired for  the  apparatus  to  sink  is  taken  as  the  consistency. 

Penetration  Test.  For  determining  the  consistency 
of  solid  or  semi-solid  bitumens,  the  penetration  into  the 
bitumen  of  a  standard  needle,  under  a  constant  weight, 
is  measured.  The  tests  are  made  at  standard  tem- 
perature and  for  definite  times.  Methods  of  making 
the  test  for  road  bitumens  are  the  same  as  those  employed 
for  asphalt  paving  mixtures,  which  are  described  in  Art.  69. 

The  American  Society  for  Testing  Materials  has 
recommended*  the  following  as  standards  for  this  test: 

"  The  penetration  of  bitumen  shall  be  the  distance 
expressed  in  hundredths  of  a  centimeter  that  a  No.  2 
needle  will  penetrate  into  it  vertically  without  friction  at 
25°  C.  under  a  stated  weight  applied  for  a  stated  length 
of  time,  the  factors  of  weight  and  time  being  determined 
as  follows: 

"  The  material  shall  first  be  tested  for  five  seconds 
under  a  weight  of  100  grams.  If  this  results  is  less  than 
ten,  the  penetration  shall  be  determined  under  a  weight 
of  200  grams  applied  for  one  minute;  if  between  10 
and  300,  the  penetration  shall  be  determined  under  a 
weight  of  100  grams  applied  for  5  seconds;  if  greater 
than  300,  the  penetration  shall  be  determined  under  a 
weight  of  50  grams  applied  for  5  seconds.  In  every 
case  the  factor  of  weight  and  time  shall  be  stated  when 
reporting  the  penetration,  and  whenever  possible  to 
obtain  both  readings,  the  penetration  under  a  loo-gram 
weight  for  5  seconds  and  under  the  modified  weight  and 
time  shall  both  be  reported.  When  testing  material 
softer  than  100  penetration,  a  containing  -receptacle  not 
less  than  ij  inches  in  diameter  shall  be  used. 

*  Proceedings,  American  Society  for^Testing  Materials  ,Vol.  XI,  p. 247. 


190      A   TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

"It  is  recommended  that  the  penetration  may  be 
determined  at  o°  C.  (32°  F.)  and  46°  C.  (114.8°  F.)  in 
addition  to  the  25°  C.  (77°  F.)  test." 


EVAPORATION  TEST. 

This  test  is  made  for  the  purpose  of  determining  the 
extent  to  which  the  material  will  give  off  the  lighter 
hydrocarbons  when  heated.  The  following  method  of 
making  the  test  is  recommended  by  the  American  Society 
for  Testing  Materials: 

"  The  loss  on  heating  of  oil  and  asphaltic  compounds 
shall  be  determined  in  the  following  manner:  Twenty 
grams  of  the  water-free  material  shall  be  placed  in  a 
circular  tin  box  with  vertical  sides,  measuring  about 
2  cm.  in  depth  by  6  cm.  in  diameter,  internal  measure- 
ment. The  penetration  of  the  material  to  be  examined 
shall,  if  possible,  be  determined  at  25°  C.  and  the  exact 
weight  of  the  sample  ascertained.  The  sample  in  the 
tin  box  shall  then  be  placed  in  a  hot-air  oven  (New  York 
Testing  Laboratory  oven  without  fan),  heated  to  163°  C. 
(325°  F.)  and  kept  at  this  temperature  for  5  hours.  At 
no  time  shall  the  temprature  of  this  oven  vary  more  than 
2°  C.  from  163°  C.  When  the  sample  is  cooled  to  normal 
temperature,  it  shall  be  weighed  and  the  percentage 
of  loss  by  volatilization  reported.  The  penetration  of 
the  residue  shall  then,  if  possible,  be  determined  at  25°  C. 
as  upon  the  original  material,  and  the  loss  in  penetration 
found  by  subtracting  this  penetration  from  the  penetra- 
tion before  heating.  In  preparing  the  residue  for  the 
penetration  test  it  shall  first  be  heated  and  thoroughly 
stirred  while  cooling." 

When  this  test  is  applied  to  solid  bitumens,  it  is  intended 
to  indicate  the  possibility  of  changes  taking  place  in  the 


BITUMINOUS    MACADAM   ROADS.  IQI 

character  of  the  bitumen  through  evaporation,  during 
application  or  subsequent  to  use  in  the  road.  It  is  com- 
mon, when  so  using  the  test,  to  make  penetration  test 
of  the  residue  from  the  evaporation  test  as  well  as  of  the 
original  bitumen. 

MELTING  POINT. 

The  temperatures  at  which  bituminous  materials 
become  sufficiently  soft  to  flow,  are  frequently  deter- 
mined and  are  known  as  the  melting-points  of  the 
materials. 

The  following  method  of  making  the  test  is  recom- 
mended by  a  Committee  of  the  American  Society  of  Civil 
Engineers: 

Melting-point  of  Residue  from  Evaporation.  The  mate- 
rial whose  melting-point  is  to  be  determined,  is  melted 
and  poured  into  a  mold  that  will  make  a  n-inch  cube.  A 
No.  10  gauge  wire  about  6  inches  to  8  inches  long  is  bent 
at  right  angles  for  a  length  of  }  inch  at  one  end  and  the 
center  of  the  cube  is  placed  on  this  end  so  that  one  of  the 
diagonals  of  the  vertical  face  of  the  cube  is  parallel  to  the 
long  part  of  the  wire.  Take  a  bottle  of  a  size  about  2 
inches  in  diameter  and  4  inches  high  and  place  a  piece 
of  white  paper  in  the  bottom  of  it.  Pass  the  long  part 
of  the  wire  through  the  cork  of  the  bottle  so  that  the  lower 
edge  of  the  cube  will  be  within  i  inch  of  the  bottom  of 
the  bottle.  Also  put  a  thermometer  through  the  cork 
so  that  the  bulb  is  opposite  the  cube.  Place  the  bottle 
in  a  water  or  oil  bath  and  raise  the  temperature  of  the 
bath  at  a  rate  of  3  to  6°  C.  a  minute.  The  melting- 
point  of  the  material  is  the  temperature  of  the  thermom- 
eter inside  the  bottle  at  the  time  that  the  material 
touches  the  paper  in  the  bottom  of  the  bottle. 


192         A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 
PARAFFINS  TEST. 

For  the  determination  of  the  amount  of  paraffine 
scale  present  in  bitumens,  the  following  method  is  usually 
employed : 

Paraffine.  One  hundred  grams  or  less  of  the  "com- 
pound is  distilled  rapidly  in  a  retort  to  dry  coke. 

Five  grams  of  the  well-mixed  distillate  is  treated  in  a 
2-ounce  flask  with  25  c.c.  Squibbs  absolute  ether;  after 
mixing  thoroughly,  25  c.c.  Squibbs  absolute  alcohol  is 
added  and  the  flask  packed  closely  in  a  freezing  mixture 
of  finely  crushed  ice  and  salt  for  at  least  30  minutes. 
Filter  the  precipitate  quickly  by  means  of  a  suction  pump, 
using  a  No.  575  C.  S.  &  S.  Q-cm.  hardened  filter 
paper.  Rinse  and  wash  the  flask  and  precipitate  (with 
i  to  i  Squibbs  alcohol  and  ether  mixture  cooled  to  —17° 
C.)  until  free  from  oil  (50  c.c.  of  wahing  solution  is 
usually  sufficient).  When  sucked  dry  remove  paper, 
transfer  waxy  precipitate  to  small  glass  dish,  evaporate 
on  steam  bath  and  weigh  paraffine  remaining  on  dish. 

Calculation.  Weight  of  paraffine  divided  by  weight  of 
distillate  taken  and  multiplied  by  per  cent  of  total  dis- 
tillate used  from  original  sample,  equals  per  cent  of 
paraffine. 

ART.  49.     SURFACE  TREATMENT. 

The  kinds  of  bitumen  employed  and  the  details  of 
application  vary  considerably  in  the  practice  of  different 
engineers  who  construct  roads  by  this  method.  In  all 
cases,  it  is  insisted  that,  before  the  application  of  the 
treatment,  the  surface  of  the  macadam  road  shall  be  in 
smooth,  firm  condition,  free  from  dust  and  dirt.  Where 
heavy  oils,  or  tars,  are  being  used,  dust  will  prevent 
penetration  into  the  surface  of  the  road,  and  holes,  or 


BITUMINOUS  MACADAM  ROADS.  193 

depressions,  will  cause  an  accumulation  of  oil,  resulting 
in  soft  spots  in  the  surface. 

The  method  most  largely  employed  is,  after  cleaning 
the  road  surface  of  dust,  to  spread  a  coating  of  hot  oil 
or  tar  over  the  surface  which  is  then  covered  with  a  layer 
of  sand  or  stone  chips  to  absorb  the  surplus  bitumen  and 
rolled  to  a  smooth  surface.  The  tar  or  oil  is  frequently 
spread  by  the  use  of  flat-nosed  watering  pots,  or  ladles, 
or  sometimes  by  the  use  of  hose  attached  to  the  kettles. 
The  bitumen  is  then  broomed  into  the  surface  in  order 
to  secure  a  uniform  coating.  This  is  accomplished  by 
laborers  with  stiff  brooms,  who  follow  the  distributors, 
and  brush  any  excess  of  oil  or  tar  upon  uncovered  spaces. 

On  large  work,  it  is  more  economical  to  distribute 
the  bitumen  from  tank  wagons,  in  which  the  oil  is  heated 
and  which  are  arranged  with  various  forms  of  distributors 
to  apply  the  hot  oil  to  the  surface.  It  is  necessary  that 
a  uniform  pressure  be  maintained  on  the  distributors, 
and  a  number  of  devices  have  been  tried  for  this  purpose, 
most  of  which  have  not  been  used  to  sufficient  extent  to 
fully  demonstrate  their  value.  The  most  successful 
distributors  seem  to  be  those  in  which  the  bitumen  is 
sprayed  upon  the  road  by  compressed  air,  and  a  number 
of  different  appliances  of  this  kind  are  now  on  the  market. 
Some  of  these,  while  capable  of  making  a  good  distribu- 
tion of  oil,  are  liable  to  quickly  get  out  of  order,  and  in 
some  instances  it  has  been  found  more  economical  to 
distribute  by  means  of  hose  attached  to  the  tank  wagons. 
For  this  purpose,  a  nozzle  which  atomizes  the  oil  is  desir- 
able, the  distribution  being  made  under  a  constant  pres- 
sure. 

After  the  oil  has  been  spread,  it  is  desirable  that  it 
be  allowed  several  hours  for  absorption  into  the  road 
surface  before  applying  the  layer  of  chips.  Very  com- 


194      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

monly,  however,  the  chips  are  immediately  applied,  and 
rolled.  Sometimes  a  second  or  even  a  third  application 
of  bitumen  and  chips  is  made  to  the  surface,  producing 
a  layer  of  oiled  material  on  top  of  the  macadam  surface. 
When  such  is  the  case  the  material  used  must  possess 
sufficient  stability  not  to  become  soft  in  wet  weather. 

The  bitumen  used  for  surface  treatment  may  be  petro- 
leum residuum,  or  tar,  so  viscous  as  to  require  heating 
before  application.  The  heating  is  commonly  done  in 
kettles  or  tanks,  mounted  on  wheels.  A  temperature 
of  about  100°  C.  is  usually  required,  although  some- 
times harder  bitumens  are  employed,  for  which  somewhat 
higher  temperatures  are  necessary. 

The  quantity  of  bitumen  required  for  a  surface  treat- 
ment with  heavy  oil  depends  upon  the  character  of  the 
road  metal,  and  extent  to  which  the  oil  may  be  absorbed, 
as  well  as  upon  the  method  of  treatment  adopted.  The 
quantities  used  vary  from  about  i  to  f  gallon  per  square 
yard  of  surface,  and  the  cost  from  about  5  cents  to  12 
cents  per  square  yard. 

The  method  of  surface  treatment  used  in  the  construc- 
tion of  telford  streets  in  St.  Louis  by  Street  Commis- 
sioner Travilla  is  shown  by  the  following  extracts  from 
his  1911  specifications: 

MACADAM  OR  SECOND  COURSE. 

"  When  the  telford  foundation  has  thus  been  formed, 
there  shall  be  spread  a  layer  of  clean,  hard  limestone 
macadam,  free  from  clay,  earth  or  rubbish;  which 
layer,  when  thoroughly  compacted,  shall  be  4  inches 
in  depth.  The  stone  shall  be  so  broken  that  all  will 
pass  a  3 -inch  ring  and  none  will  pass  a  ij-inch  ring. 
The  stone  shall  be  broken  to  conform  to  the  above  re- 
quirements before  being  brought  on  the  line  of  the  work. 


BITUMINOUS  MACADAM  ROADS.  IQ5 

This  course  shall  be  thoroughly  consolidated  by  a  roller, 
as  above  specified,  and  any  unsvenness  in  the  surface 
shall  be  corrected  before  spreading  the  limestone  screen- 
ings. The  macadam  course  having  been  finished,  the 
interstices  of  the  stones  shall  be  completely  filled  with 
clean  macadam  screenings,  containing  50  per  cent  dust. 
This  layer  shall  then  be  flooded  and  rolled  until  it  is 
compact  and  solid,  and  ceases  to  creep  under  the  action 
of  the  roller.  Rolling  shall  be  continued  until  the 
screenings  and  water  flush  to  the  surface  upon  all  parts 
of  the  roadway.  The  surface  of  the  macadam  shall  be 
broomed  immediately  after  rolling,  leaving  the  clean 
stone  projecting;  the  voids,  however,  to  be  thoroughly 
filled. 

ROAD  OIL. 

"  When  the  macadam  surface,  as  above  prepared, 
has  thoroughly  dried  out  to  the  satisfaction  of  the  street 
commissioner,  hot  road  oil  shall  be  spread  over  the  same 
to  the  amount  of  J  gallon  per  square  yard  of  surface. 
The  oil,  as  below  specified,  shall  be  applied  to  the  road- 
way surface  at  a  temperature  of  at  least  250°  F.  Proper 
sand  barricades  shall  be  constructed  along  the  edge  of 
the  granitoid  gutters  to  prevent  the  oil  from  flowing  into 
the  same.  If  the  street  commissioner  deems  it  neces- 
sary, he  may  require  the  macadam  surface  to  be  thor- 
oughly hand-broomed  and  to  be  lightly  sprinkled  with 
water  before  the  oil  is  applied. 

"The  specific  gravity  of  the  road  oil  shall  not  be  less 
than  0.959  (16.0°  Be.  at  60°  F.). 

'The  loss  upon  heating  a  20-gram  sample  of  the  oil 
in  a  vessel  2\  inches  in  diameter  and  ij  inches  high, 
with  vertical  sides,  for  5  hours  at  325°  F.  (163°  C.) 
shall  not  exceed  3  per  cent  by  weight.  It  shall  not  con- 


196      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

tain  more  than  \  per  cent  by  weight  of  matter  insoluble 
in  carbon  bisulphide." 

The  following  method  of  treating  the  surfaces  of 
finished  macadam  roads  is  given  *  by  Mr.  Charles  W. 
Ross,  as  used  at  Newton,  Mass: 

"  Several  macadam-surfaced  streets,  having  varying 
grades  up  to  a  maximum  of  9  per  cent,  and  subjected  to 
heavy  horse-drawn  and  automobile  traffic,  were  selected 
for  the  liquid  asphalt  treatment.  The  method  used  was 
as  follows :  A  quantity  of  sand  was  heated  to  a  temperature 
of  about  200  °  F.,  dumped  in  a  pile,  and  leveled.  The 
asphalt  was  poured  over  the  hot  sand  in  the  proportion 
of  i  gallon  to  each  cubic  yard  of  sand,  and  then  the  whole 
mass  was  turned  with  shovels  or  mixed  in  a  concrete 
mixer.  (The  latter  being  preferable  on  account  of  cost.) 
This  work  was  done  at  the  pit.  The  mixture  was  teamed 
to  the  work  and  spread  on  the  roadway  to  a  depth  of 
less  than  -J-  inch,  being  raked  even  with  i4-tooth  wooden 
rakes.  Rolling  was  not  considered  necessary  and  the 
street  was  kept  open  for  traffic  at  all  times.  The  cost 
of  this  treatment  was  about  3  cents  per  square  yard. 
It  has  the  advantage  of  leveling  and  building  up  the 
surface  of  the  road,  each  new  application  providing  a 
new  wearing  surface.  This  work  has  remained  in 
perfect  condition  without  further  expense  since  the  summer 
of  1909." 

Surface  treatment  of  macadam  roads  with  bituminous 
materials  has  been  quite  largely  used  in  the  United  States 
during  the  past  few  years.  In  general,  when  road  sur- 
faces in  good  condition  have  been  properly  treated 
with  good  materials,  the  results  have  been  fairly  satis- 
factory, particularly  when  required  to  resist  automobile 
traffic.  In  some  instances,  however,  these  roads  do  not 
*  Transactions,  American  Society  of  Civil  Engineers,  Vol.  LXXIII,  p.  47. 


BITUMINOUS  MACADAM  ROADS.  197 

seem  so  satisfactory  when  the  traffic  is  mainly  horse- 
drawn,  which  traffic  may  cut  through  the  oiled  layer 
upon  the  surface  and  cause  the  road  to  rut. 

ART.  50.     PENETRATION  METHOD. 

Roads  constructed  by  the  penetration  methods  by 
different  engineers  vary  considerably  in  details.  In 
most  instances,  the  bottom  course  is  constructed  in  the 
same  manner  as  for  water-bound  macadam,  the  voids 
being  completely  filled  with  sand  or  screenings,  and  rolled 
to  a  hard  surface.  After  completing  the  foundation 
course,  the  second  course  of  road  metal  is  placed.  This 
may  consist,  when  hard  rock  is  employed,  of  stone  vary- 
ing in  size  from  about  \  inch  to  ij  inches,  or,  when 
limestone  is  used,  from  about  i  inch  to  2\  inches  in 
diameter,  the  road  metal  is  placed  to  the  proper  depth 
(usually  2\  or  3  inches)  and  rolled  to  an  even  surface. 
The  hot  bitumen  is  then  poured  over  the  surface,  and 
allowed  to  run  into  and  coat  the  stone  composing  the 
surface  course.  In  some  instances  a  light  coating  of 
stone  chips  is  used,  to  partially  fill  the  voids,  before  the 
bitumen  is  applied.  The  bitumen  must  be  quite  fluid 
at  the  temperature  of  application,  in  order  to  run  into 
the  surface  material  and  thoroughly  coat  the  stone, 
About  ij  gallons  of  bitumen  per  square  yard  are  usually 
required.  The  surface  is  next  covered  with  a  coating 
of  stone  chips,  which  are  rolled  into  the  surface.  Upon 
completion  of  the  rolling,  any  surplus  fine  material 
should  be  swept  from  the  surface  before  adding  the 
paint  coat,  which  consists  in  pouring  about  \  or  \  gallon 
of  bitumen  per  square  yard  over  the  surface,  covering 
with  screenings,  and  rolling  to  a  finished  surface. 

The  bitumen  may  be  applied  to  the  surface  by  the  same 


198      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

methods  as  are  used  in  surface  treatments,  although 
some  of  the  automatic  distributors  do  not  work  so  well 
over  these  comparatively  rough  surfaces  as  on  the  fin- 
ished macadam. 

The  method  of  construction  used  by  the  Illinois  High- 
way Commission  is  shown  by  the  following  extract  from 
their  1911  specifications: 

SPREADING  SECOND  COURSE  OF  STONE. 

"  The  broken  stone  for  the  second  course  is  not  to  be 
spread  before  the  first  course  has  been  completed  and 
shoulders  made  as  herein  specified.  The  second  course 
shall  be  of  2^-inch  stone  and  shall  be  spread  to  compact 
under  rolling  to  the  thickness  shown  on  the  plans. 

HARROWING  SECOND  COURSE  OF  STONE. 

"  After  the  second  course  of  stone  has  been  spread, 
it  shall  be  harrowed  as  hereinbefore  described  for  the 
first  course  of  stone.  (See  p.  148). 

ROLLING  SECOND  COURSE  OF  STONE. 

"  After  the  broken  stone  for  the  second  course  has 
been  spread  and  harrowed  to  the  required  thickness  and 
has  a  proper  cross-section,  it  is  to  be  rolled  with  a  steam 
roller,  weighing  not  less  than  10  tons,  until  it  is  compacted 
to  form  a  firm,  smooth  surface.  The  rolling  is  to  begin 
at  the  sides,  the  shoulders  for  a  width  of  at  least  5  feet 
first  being  rolled  until  firm.  When  completed,  the  sur- 
face of  the  shoulders  and  of  the  second  course  of  broken 
stone  must  be  smooth  and  continuous  with  a  cross  slope 
as  shown  on  the  plans. 


BITUMINOUS  MACADAM  ROADS.  199 

UNEVENNESS  OR  DEPRESSIONS. 

"  If  any  unevenness  or  depressions  appear  during 
or  after  the  rolling  of  the  second  course,  either  on  the 
surface  of  the  shoulder  or  the  broken  stone,  suitable 
material  shall  be  added  to  remove  all  such  unevenness  or 
depressions,  earth  being  used  on  the  shoulders  and  stone 
for  the  macadam. 

SPREADING  CHIPS. 

"  After  the  second  course  of  stone  has  been  rolled  and 
completed  as  specified,  the  surface  voids  are  to  be  filled 
with  chips,  free  from  dust,  which  shall  be  whipped  into 
the  surface  from  shovels,  the  quantity  being  such  as  will 
just  fill  these  surface  voids.  After  the  chips  have  been 
whipped  into  the  surface,  it  shall  be  gone  over  with  a 
stiff  brush  broom  and  all  chips  remaining  on  the  surface 
of  the  stone  swept  into  the  voids  in  the  surface,  and  if  an 
excess  remains  after  the  voids  have  been  filled,  they  shall 
be  swept  off  to  the  edge  of  the  macadam. 

SPREADING  BITUMINOUS  BINDER. 

"  Upon  the  surface  prepared  as  above  described, 
there  shall  be  uniformly  distributed  i  gallon  per  square 
yard  of  surface  of  the  asphalt  binder  herein  specified. 
The  asphalt  binder  shall  be  applied  at  a  temperature 
not  less  than  350°  F.,  and  shall  be  spread  on  the  surface 
in  a  manner  which  will  insure  that  a  uniform  amount  is 
applied  to  all  parts  of  it. 

ROLLING  BINDER. 

"  Immediately  after  the  first  course  of  binder  is  spread, 
the  surface  is  to  be  rolled,  preferably  with  a  tandem 


200      A  TEXT-BOOK  ON  ROADS   AND   PAVEMENTS. 

roller,  weighing  not  less  than  8  tons.  The  roller  must 
be  provided  with  means  to  keep  the  surface  of  the  wheels 
sprinkled  with  water.  The  rolling  is  to  continue  until 
the  surface  has  become  hard,  smooth  and  as  closely  com- 
pacted as  possible. 

SPREADING  SECOND  COURSE  OF  CHIPS. 

"  After  the  first  course  of  binder  has  been  spread  and 
rolled  as  above  specified,  there  shall  be  spread  a  quantity 
of  chips  which  will  be  just  sufficient  to  fill  the  remaining 
voids  in  the  surface.  The  chips  shall  be  brushed  into  the 
voids  with  a  stift  brush  broom. 

SPREADING  PAINT  COAT  OF  ASPHALT  BINDER. 

"  After  the  second  course  of  chips  has  been  spread 
and  brushed  into  the  surface,  a  second  course  of  the 
approved  binder  shall  be  spread  at  the  rate  of  \  gallon 
per  square  yard.  The  binder  shall  be  spread  at  a  tem- 
perature of  not  less  than  350°  F.  and  shall  be  applied  in 
such  a  manner  as  will  insure  that  a  thin  even  coat  of  the 
binder  covers  the  entire  surface. 

DUSTING  AND  ROLLING  FINISHED  SURFACE. 

"  After  the  paint  coat  of  the  bituminous  compound 
has  been  spread,  the  surface  shall  be  dusted  lightly  with 
coarse  quartz  sand,  not  to  exceed  one  (i)  cubic  yard 
per  300  square  yards  of  surface,  and  the  surface  rolled 
with  a  10- ton  roller,  the  wheels  being  wet  to  prevent 
sticking.  After  the  surface  has  been  rolled,  it  shall 
be  allowed  to  stand  for  one-half  day  before  being  opened 
to  traffic." 


BITUMINOUS  MACADAM  ROADS.  2OI 

The  method  recommended  by  the  Association  for 
Standardizing  Paving  Specifications  is  shown  by  the 
following  extract  from  proposed  specifications: 

"  Upon  the  bottom  course  shall  be  evenly  spread 
crusher  run  stone  which  shall  pass  a  3 -inch  ring  and 
be  retained  upon  a  i-inch  screen,  to  a  finished  depth 
of  two  and  one-half  (2^-)  inches.  This  course  shall  be 
dry  rolled  with  a  steam  roller  herein  before  mentioned 
only  until  the  individual  fragments  have  keyed  together, 
the  surface,  while  even  and  conforming  to  the  required 
crown,  being  left  open  or  porous  in  order  to  allow  the 
penetration  of  the  hot  bituminous  binder. 

"  The  binder  shall  be  heated  in  an  approved  heater 
equipped  with  a  fixed  or  portable  thermometer  which 
will  clearly  a  ad  accurately  indicate  the  temperature  of 
the  binder.  The  bituminous  binder  shall  be  heated  to  a 
temperature  of  not  less  than  250°  F.,  nor  more  than  350° 
F.,  and  shall  be  uniformly  distributed  over  the  macadam 
by  suitable  appliances  at  a  rate  of  not  less  than  one  and 
seven-tenths  (1.7)  gallons  to  the  square  yard.  Directly 
after  application,  clean  trap  rock,  or  equally  satisfactory 
stone  chips,  free  from  dust  and  consisting  of  fragments 
which  will  pass  a  i-inch  ring  but  be  retained  upon  a 
three-eighths  (^)  inch  screen,  shall  be  spread  over  the 
surface  in  sufficient  quantities  to  fill  the  surface  voids 
and  prevent  the  binder  from  sticking  to  the  wheels 
of  the  roller.  Care  shall  be  exercised  not  to  apply  more 
stone  chips  than  will  just  fill  the  interstices  and  an> 
surplus  material  shall  be  swept  from  the  surface,  as 
directed.  The  road  shall  then  be  rolled  until  solid, 
more  stone  chips  or  screenings  being  applied  as  required 
in  order  to  maintain  satisfactory  conditions. 

"  A  seal,  flush,  paint  or  squeegee  coat  of  the  hot 
binder  shall  be  uniformly  distributed  over  the  surface  at 


202         A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

a  rate  of  at  least  one-half  (J)  gallon  to  the  square  yard. 
Clean  stone  chips  or  screenings  such  as  previously 
described  shall  then  be  spread  over  this  seal  coat  in  just 
sufficient  quantity  to  take  up  all  excess  of  binder  and 
form  a  smooth  well-bonded  surface  when  rolled.  The 
road  shall  be  rolled  until  smooth  and  firm  and  to  the 
proper  lines  and  grades. 

"  The  stone  must  be  dry  and  free  from  dirt  or  dust  at 
the  time  of  applying  the  bituminous  binder.  The  applica- 
tion of  binder  shall  not  be  made  when  the  atmospheric 
temperature  is  below  50°  F.  unless  specially  permitted 
by  the  engineer." 

ART.  51.    MIXING  METHOD. 

The  construction  of  bituminous  macadam  by  the 
mixing  method  differs  from  that  by  the  penetration 
method,  in  that  the  bitumen  is  mixed  with  the  aggregate 
before  it  is  placed  upon  the  road.  The  lower  course 
is  formed  in  the  usual  manner,  and  finished  as  in  water- 
bound  macadam.  The  hot  bitumen  is  mixed  with  broken 
stone,  which  is  also  usually  heated,  thus  permitting  the 
use  of  bitumen  of  harder  consistency  than  could  be  used 
for  pouring  or  for  mixing  with  cold  stone.  The  quantity 
of  bitumen  used  is  commonly  somewhat  less  than  with 
the  penetration  method,  and  should  be  sufficient  to 
thoroughly  coat  all  the  stones  of  the  aggregate. 

It  is  desirable  that  the  stone  be  so  graded  as  to  reduce 
the  voids  to  a  minimum,  and  sometimes  different  sizes 
of  rock  are  mixed  for  this  reason.  Usually,  however, 
in  macadam  work,  careful  grading  of  sizes  would  be  too 
expensive,  and  the  material  is  used  between  certain  limits 
of  size  as  it  comes  from  the  crusher,  as  in  the  other  methods. 
In  the  construction  of  city  pavements  by  the  Bitulithic 
process,  careful  grading  of  stone  into  several  sizes,  and 


BITUMINOUS  MACADAM  ROADS.  203 

exact  mixing  to  secure  best  results,  is  practiced.  This 
will  be  considered  in  Art.  73. 

In  constructing  roads  by  this  method,  the  mixing  is 
done  either  in  mechanical  mixers  or  by  hand.  On  large 
work,  mechanical  mixing  is  usually  cheaper  and  more 
satisfactory.  A  number  of  portable  mixers  are  now  on 
the  market,  several  of  which  are  capable  of  doing  excellent 
work.  In  hand  mixing,  the  cost  depends  upon  whether 
the  plant  is  arranged  for  convenient  handling  of  the 
materials,  with  the  least  amount  of  labor.  The  stone, 
in  hand  mixing,  is  commonly  used  cold,  or  it  may  be 
heated  sufficiently  to  prevent  the  sudden  chilling  of 
the  bitumen  when  it  comes  in  contact  with  the  stone. 
The  bitumen  employed  should  be  a  heavy  viscous  mate- 
rial, sufficiently  soft  to  flow  slightly  at  the  temperature 
of  the  stone. 

When  the  mixing  of  the  bitumen  and  stone  is  com- 
pleted, the  concrete  is  placed  upon  the  road  to  the  desired 
thickness  and  rolled.  Sometimes  a  layer  of  stone  chips 
is  added  before  rolling  to  prevent  sticking.  A  paint 
coat  is  then  usually  applied  to  the  surface,  and  covered 
with  a  light  coating  of  screenings,  and  the  whole  rolled 
to  a  finished  surface. 

Bituminous  macadam,  constructed  by  the  mixing 
method,  has  been  extensively  used  in  England,  where  it 
seems  to  be  commonly  preferred  to  the  penetration,  or 
tar  grouting,  method.  The  material  used  in  England 
is  almost  exclusively  tar.  In  many  instances,  tar  concrete 
is  prepared  at  a  central 'plant,  located  conveniently  to 
the  supply  of  aggregate,  and  shipped  ready  for  use  to  the 
site  of  the  work.  A  complete  discussion  of  English 
practice  in  the  construction  of  tar  macadam  roads  is 
given  by  Mr.  J.  Walker  Smith.* 

*  Dustlcss  Roads,  Tar  Macadam,  London,  1909. 


204     A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  52.     SELECTION  OF  BITUMINOUS  MATERIALS. 

The  selection  of  bitumens  for  use  on  macadam  roads 
always  depend  largely  upon  local  conditions.  The  kinds 
of  material  which  are  most  readily  available;  the  char- 
acter of  the  road  metal  to  be  employed  and  method  of 
construction  to  be  adopted;  the  local  conditions  of  climate 
and  traffic  which  the  road  is  to  meet,  are  all  to  be  con- 
sidered before  a  proper  selection  can  be  made. 

The  varying  results  which  have  been  obtained  with  the 
same  materials  in  different  localities,  and  under  differing 
conditions,  make  it  impracticable,  at  present,  to  establish 
any  standard  specifications  for  such  materials.  Expe- 
rience in  the  use  of  these  materials  is,  however,  rapidly 
shaping  practice,  and  determining  the  most  suitable 
materials  for  localities  in  which  bituminous  macadam 
construction  is  being  used. 

The  Illinois  Highway  Commission  uses  the  penetra- 
tion method  in  constructing  bituminous  macadam, 
applying  the  bituminous  binder  at  a  temperature  of  about 
350°  F.  The  bitumens  used  are  petroleum  residuums, 
or  oil  asphalts,  and  the  1911  specifications  are  as  follows: 

ASPHALT  BINDER. 

"  The  asphalt  binder  used  shall  conform  to  the  follow- 
ing specifications.  The  various  properties  herein  de- 
scribed to  be  determined  by  the  methods  proposed  by  the 
American  Society  for  Testing  Materials. 

"Specific  Gravity.  The  asphalt  shall  have  a  specific 
gravity  not  less  than  0.97. 

"Total  Bitumen.  The  asphalt  shall  be  soluble  in  cold 
carbon  bisulphide  to  the  extent  of  at  least  98  per  cent. 

"Naphtha  Insoluble  Bitumen.    Of  the  total  bitumen  not 


BITUMINOUS  MACADAM   ROADS.  205 

less  than  20  per  cent  nor  more  than  25  per  cent  shall 
be  insoluble  in  86°  Be.  naphtha. 

"Loss  on  Evaporation.  When  20  grams  (in  a  tin  dish 
2j  inches  in  diameter,  with  vertical  sides)  are  maintained 
at  a  temperature  of  170°  C.  for  5  hours  in  a  N.  Y.  testing 
laboratory  oven,  the  evaporation  loss  shall  not  exceed 
2  per  cent  and  the  penetration  shall  not  have  been  de- 
creased more  than  25  per  cent. 

"Fixed  Carbon.  The  fixed  carbon  shall  not  exceed  12 
per  cent  by  weight. 

"Penetration.  The  penetration  as  determined  with  the 
Dow  machine  using  a  No.  2  needle,  loo-gram  weight, 
5  sec.  time  and  a  temperature  of  25°  C.,  shall  not  be  less 
than  5.0  mm.  nor  more  than  10.0  mm. 

"Paraffine.  The  asphalt  shall  not  contain  to  exceed 
2  per  cent  by  weight  of  paraffine." 

In  the  construction  of  bituminous  macadam  by  the 
penetration  method  in  Ohio,  by  Mr.  James  R.  Marker, 
State  Highway  Commissioner,  the  bitumen  is  applied 
at  temperature  between  250°  F.  and  350°  F.  Several 
kinds  of  bitumen  may  be  employed,  each  of  which  is 
provided  for  in  the  specifications,  which  are  as  follows: 

OIL  ASPHALT. 

"  i.  The  oil  asphalt  shall  have  a  specific  gravity  of 
not  less  than  c  965  at  25°  C. 

"2.  It  shall  be  soluble  in  o.p.  carbon  disulphide  at 
air  temperature  to  at  least  99.5  per  cent  and  shall  con- 
tain not  over  0.3  per  cent  organic  matter  insoluble. 

"3.  It  shall  contain  not  less  than  18  per  cent  nor  more 
than  25  per  cent  of  bitumen  insoluble  in  86°  Be.  paraffine 
naphtha  at  air  temperature. 

"  4.  When  tested  for  5  seconds  at  25°  C.  with  a  standard 


206      A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

No.  2  needle  weighted  with  100  grams,  it  shall  show  a 
penetration  of  not  less  than  10  mm.  nor  greater  than  20 
mm. 

"  5.  When  20  grams  of  the  material  is  heated  for  5 
hours  in  a  cylindrical  dish  approximately  2\  inches  in 
diameter  by  f  inch  high,  at  a  constant  temperature 
of  163°  C.,  the  loss  in  weight  by  volatilization  shall  not 
exceed  3  per  cent.  The  residue  remaining  shall  show  a 
penetration  of  not  less  than  6  mm.  when  tested  in  the 
manner  hereinbefore  described. 

"  6.  Its  fixed  carbon  shall  be  not  less  than  7.5  per  cent. 

"  7.  The  oil  asphalt  upon  delivery  shall  be  free  from 
water. 

FLUXED  NATIVE  ASPHALT. 

"  i.  The  fluxed  asphalt  shall  have  a  specific  gravity 
of  not  less  than  i.oi  at  a  temperature  of  25°  C. 

"2.  Its  solubility  at  air  temperature  in  c.p.  carbon 
disulphide  for  the  following  named  materials,  or  materials 
similar  thereto,  shall  be  at  least  95  per  cent  for  Bermudez 
products,  8 1  per  cent  for  Cuban  products  and  66  per 
cent  for  Trinidad  products. 

3.  It  shall  contain  not  less  than  18  per  cent  nor  more 
than  25  per  cent  of  bitumen,  insoluble  in  86°  Be. 
paraffme  naphtha. 

"4.  It  shall  yield  not  less  than  9  per  cent  nor  more 
than  13  per  cent  of  fixed  carbon. 

"  5.  The  penetration  shall  be  between  10  and  20  mm. 
when  tested  for  5  seconds  at  25°  C.  with  a  No.  2  needle 
weighted  with  100  grams. 

"  6.  When  20  grams  of  the  material  is  heated  for  5 
hours  in  a  cylindrical  dish  approximately  z\  inches  in 
diameter  by  f  inch  high  at  a  constant  temperature  of 
163°  C.,  the  loss  in  weight  shall  not  exceed  5  per  cent. 


BITUMINOUS  MACADAM   ROADS.  207 

The  residue  thus  obtained  shall  show  a  penetration  of 
not  more  than  12  mm.,  nor  less  than  5  mm.  when  tested 
in  the  manner  hereinbefore  described. 

"  7.  The  fluxed  asphalt  upon  delivery  shall  be  free 
from  water. 

REFINED  COAL  TAR. 

"  i.  The  tar  shall  have  a  specific  gravity  of  not  less 
than  1.170  nor  greater  than  1.250  at  25°  C. 

"  2.  On  extraction  with  carbon  disulphide,  it  shall 
contain  not  more  than  20  per  cent  free  carbon. 

"3.  Upon  ignition  it  shall  show  not  over  0.5  per  cent 
inorganic  residue. 

"  4.  When  the  sample  of  tar  is  submitted  to  the  float 
test,*  as  hereinafter  described,  the  float  shall  sink  in  water 
maintained  at  50°  C.  in  not  less  than  2\  minutes  nor  more 
than  3  minutes. 

"  5.  When  250  c.c.  of  the  tar  is  distilled  in  a  75o-c.c. 
glass  retort  at  a  rate  not  exceeding  2  drops  of  distillate 
per  second,  the  total  distillate  to  170°,  as  registered  by  a 
thermometer  whose  bulb  is  level  with  the  bottom  of  the 
outlet  of  the  body  of  the  retort,  shall  not  exceed  2  per 
cent  by  volume  of  the  original  material.  The  total  dis- 
tillate to  270°  C.  shall  in  no  case  exceed  50  per  cent  and 
when  the  tar  contains  more  than  10  per  cent  free  carbon, 
this  distillate  shall  not  exceed  40  per  cent  by  volume  of 
the  original  material. 

"  6.  The  tar  upon  delivery  shall  be  free  from  water. 

REFINED  WATER-GAS  TAR. 

"  i.  The  tar  shall  have  a  specific  gravity  of  not  less 
than  1.160  nor  more  than  1.185  at  25°  C- 

"  2.  It  shall  be  soluble  in  c.p.  carbon  disulphide  at 
air  temperature  to  at  least  95  per  cent. 
*  See  footnote  on  p.  208. 


208      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

"  3.  Upon  ignition  it  shall  show  not  over  0.5  per  cent 
inorganic  residue. 

"  4.  When  the  sample  of  tar  is  subjected  to  the  float 
test,*  as  hereinafter  described,  the  float  shall  sink  in  water 
maintained  at  32°  C.  in  not  less  than  13  minutes  nor 
more  than  18  minutes. 

"5.  When  250  c.c.  of  the  tar  is  distilled  in  a  750  c.c. 
glass  retort  at  a  rate  not  exceeding  2  drops  of  distillate 
per  second,  the  total  distillate  to  170°  C.,  as  registered 
by  a  thermometer  whose  bulb  is  level  with  the  bottom 
of  the  outlet  of  the  body  of  the  retort,  shall  not  exceed 
4  per  cent  by  volume  of  the  original  material.  The 
total  distillate  to  270°  C.  shall  not  exceed  35  per  cent 
by  volume  of  the  original  material. 

"  6.  The  tar  upon  delivery  shall  be  free  from  water." 

*The  apparatus  used  in  making  the  float  test  is  manufactured  by 
Howard  &  Morse,  Brooklyn,  N.  Y.,  and  consists  of  two  parts,  an 
aluminum  float  or  saucer,  and  a  conical  brass  collar.  The  float  contains 
an  aperture  in  the  bottom  threaded  so  as  to  receive  the  smaller  end  of 
the  brass  collar.  In  using  this  apparatus,  the  collar  is  placed  upon  a 
brass  plate  having  an  amalgamated  surface.  The  collar  is  then  filled 
with  the  bitumen,  after  it  has  been  softened  by  gentle  heating.  As  soon 
as  the  bitumen  has  cooled  sufficiently  to  handle,  it  is  placed  in  ice-water 
for  fifteen  minutes.  It  must  then  be  attached  to  the  float,  which  should 
be  immediately  placed  upon  the  surface  of  the  water,  which  is  main- 
tained at  50°  C.  for  the  coal-tar  test,  and  at  32"  C.  for  the  water-gas  tar 
test.  As  the  plug  of  bitumen  in  the  brass  collar  becomes  warm  and 
fluid,  it  is  gradually  forced  out  of  the  collar  and  the  water  gains  entrance 
to  the  saucer.  The  time  in  seconds  elapsing  between  placing  the  appara- 
tus on  the  water  and  when  the  water  gains  entrance  into  the  saucer 
should  be  determined  by  means  of  a  stop-watch. 


CHAPTER  VII. 

FOUNDATIONS  FOR  PAVEMENTS. 
ART.  53.     PREPARATION  OF  ROAD-BED. 

IN  forming  a  road-bed  upon  which  to  place  a  pave- 
ment, the  earth  should  be  brought  at  subgrade  to  the 
form  of  a  finished  road-surface,  leaving  room  for  the 
superstructure  of  uniform  thickness  to  be  placed  upon 
it.  Thorough  drainage  must  of  course  be  carefully  at- 
tended to  when  necessary.  This  has  been  already  dis- 
cussed in  Chapter  II. 

In  the  construction  of  a  road-bed  to  support  a 
pavement,  the  same  principles  are  involved  as  in  the 
earthwork  of  a  common  road,  which  has  been  discussed 
in  Art.  26,  and  the  same  methods  may  be  employed 
in  handling  the  earth.  In  grading,  the  surface  should 
be  left  high  enough  to  allow  for  the  compression  pro- 
duced in  rolling.  The  amount  of  settling  to  be  ex- 
pected under  the  roller  will  vary  with  the  character 
of  the  material  and  the  weight  of  the  roller.  With  a 
heavy  steam  roller,  the  compression  may  vary  from  J 
inch  for  stiff  soil  in  dry  condition  to  about  3  inches  for 
light  porous  soil.  The  allowance  to  be  made  can  only 
be  judged  from  experience  with  the  soil  in  question. 

The  road-bed,  after  being  brought  to  the  proper 
grade,  should  be  thoroughly  compacted  by  rolling 
before  placing  the  pavement.  Sometimes  in  the  use  of 
a  heavy  roller,  when  the  material  is  of  a  light  nature,  it 
is  shoved  forward  in  a  wave  before  the  roller  and  re- 

209 


210       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

fuses  to  become  compacted,  in  which  case  a  thin  layer 
of  gravel  or  small  stone  placed  upon  the  surface  of 
earth  before  rolling  may  have  the  effect  of  consolidat 
ing  the  road-bed  under  the  roller  to  a  hard  surface. 

The  roller  should  pass  several  times  over  the  road- 
bed. When  low  places  are  developed,  which  roll  down 
below  grade,  they  should  be  filled  and  rolled  again 
until  brought  to  proper  grade.  Passing  the  roller  trans- 
versely over  recently  filled  trenches  will  always  produce 
depressions  which  require  refilling.  Where  such  trenches 
exist,  the  rolling  should  be  very  carefully  done. 

In  rolling,  soft  spots  are  sometimes  discovered,  which 
cannot  be  compacted  by  rolling.  In  such  cases  the 
soft  material  should  be  removed  and  replaced  with 
better  material  to  a  sufficient  depth  to  admit  of  roll- 
ing the  road-bed  to  a  compact  surface. 

In  some  instances,  repeated  rolling  of  light  material 
with  a  heavy  roller  may  have  the  effect  of  working  the 
material  loose  so  that  it  moves  in  a  wave  before  the 
roller,  although  the  first  rolling  leaves  the  road-bed 
compact.  In  such  cases  it  is  desirable  to  avoid  too 
much  rolling. 

Where  much  grading  is  to  be  done,  it  is  usually  de- 
sirable to  do  the  rough  work  before  setting  the  curb 
upon  the  street,  if  a  new  curb  is  to  be  placed.  It  is, 
however,  much  easier  to  finish  the  grade  after  the  curb 
is  set,  as  a  line  across  the  street  at  the  top  of  the  curb 
is  a  convenient  means  of  getting  the  elevation  of  points 
on  the  subgrade. 

ART.  54.     TRENCHES  IN  STREETS. 

The  opening  of  trenches  for  water,  gas,  and  sewer 
pipes  in  the  streets  is  perhaps  the  greatest  cause  of  de- 


FOUNDATIONS   FOR   PAVEMENTS.  211 

struction  of  pavements  to  be  found  in  the  average 
city.  This  is  especially  true  of  the  smaller  cities, 
where  wear  from  traffic  is  not  excessive. 

In  constructing  a  pavement  in  an  unpaved  street 
an  effort  should  always  be  made  to  lay  all  pipes  which 
are  likely  to  be  needed  in  the  street  for  a  considerable 
period,  in  so  far  as  they  can  be  foreseen,  before  placing 
the  pavement.  Where  a  cut  is  made  through  a  pave- 
ment for  a  trench  it  is  a  matter  of  considerable 
difficu^  to  backfill  the  trench  and  replace  the  pave- 
ment in  as  good  condition  as  before  it  was  cut,  and 
great  care  is  required  to  prevent  the  subsequent  set- 
tlement of  the  pavement  over  the  trench.  The  filling 
of  trenches  over  which  a  pavement  is  to  be  placed 
requires  very  close  inspection,  and  frequently,  neglect 
of  such  inspection  causes  much  trouble  subse- 
quently. 

The  most  common  method  of  filling  trenches  in 
unpaved  streets  is  to  throw  the  earth  in  loosely,  and 
pile  the  surplus  earth  in  a  ridge  over  the  trench,  leaving 
it  for  the  natural  settlement,  when  wet  weather  comes, 
to  ultimately  compact  the  earth  in  the  trench. 
Usually,  the  settlement  of  such  a  trench  wrill  extend 
over  a  long  period,  and  there  is  danger  of  injury  to  a 
pavement  built  over  the  trench,  even  after  several 
months  have  elapsed  and  settlement  seems  to  have 
taken  place.  Rolling  will  compact  the  earth  in  the 
top  of  the  trench,  but  its  effect  does  not  reach  to  any 
considerable  depth  in  the  trench,  or  prevent  later 
settlement.  There  are  many  instances  in  which 
disastrous  settlements  of  pavements  have  occurred 
over  trenches,  although  the  material  in  the  trenches 
had  been  considerably  settled  by  rains  and  the  surface 
rolled  with  a  heavy  roller. 


212        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Flooding.  It  is  common  practice  to  settle  the  earth 
in  trenches  by  flooding  with  water.  This  is  accom- 
plished either  by  repeatedly  filling  a  few  inches  of 
earth  into  the  trench  and  then  saturating  with  water, 
or  by  flooding  the  trench  with  a  few  inches  of  water 
and  filling  the  earth  into  the  water.  It  is  difficult  to 
compact  the  earth  by  flooding  so  that  no  further  settle- 
ment will  take  place,  and  it  is  necessary  to  use  care  that 
the  earth  be  not  thrown  in  in  too  large  quantities  at 
once,  as  when  the  trench  is  filled  with  scrapers  or 
graders.  When  the  soil  is  clay,  subsequent  settlement 
will  take  place  as  the  clay  shrinks  upon  drying  out. 

In  filling  sewer  trenches  in  this  manner  there  is 
usually  danger  of  breaking  the  joints  of  the  sewer  in 
flooding  the  trench.  Several  instances  have  been  noted 
in  which  this  has  occurred,  and  the  practice  should 
be  avoided. 

Tamping.  The  only  method  of  effectively  compact- 
ing ordinary  earth  in  a  trench  so  that  no  danger  of 
subsequent  settlement  shall  exist  is  by  placing  the 
earth  in  thin  layers,  not  more  than  4  or  5  inches 
thick,  and  tamping  each  layer  thoroughly.  To  accom- 
plish this  the  earth  must  be  damp  enough  to  pack  well, 
but  not  too  wet. 

The  earth  compacts  into  smaller  space  when  rammed 
in  the  trench  than  it  formerly  occupied,  so  that  when 
the  pipe  is  small  as  compared  with  the  size  of  the 
trench  there  may  not  be  enough  earth  removed  in 
excavating  the  trench  to  entirely  refill  it. 

ART.  55.     PURPOSE  OF  FOUNDATION. 

The  chief  object  of  the  foundation  or  base  of  a 
pavement  is  to  distribute  the  concentrated  loads  which 


FOUNDATIONS  FOR  PAVEMENTS.  213 

come  upon  the  surface  of  the  road  over  a  greater  area 
of  the  usually  softer  and  weaker  road-bed,  in  order 
that  these  loads  may  not  produce  indentations  in  the 
surface. 

In  a  foundation  composed  of  independent  blocks 
extending  through  its  thickness,  as  in  the  case  of  a 
stone-block  pavement  in  which  the  blocks  rest  directly 
upon  the  road-bed  or  upon  a  thin  layer  of  sand,  the  load 
which  comes  upon  the  top  of  any  block  will  be  dis- 
tributed over  the  area  covered  by  the  base  of  the  block. 

Where  the  foundation  is  composed  of  small  independ- 
ent particles,  like  sand  or  loose  rounded  gravel,  with 
no  cohesion  through  the  mass,  the  pressure  is  distrib- 
uted over  the  base  of  a  cone  whose  vertex  is  in  the 
point  of  application  of  the  load,  and  the  inclination  of 
whose  elements  depends  upon  the  friction  of  the  par- 
ticles of  the  material  upon  each  other.  In  this  case 
the  area  over  which  the  load  is  distributed  varies 
directly  as  the  square  of  the  thickness  of  the  founda- 
tion. Sand,  it  is  to  be  observed,  has  also  the  property, 
when  confined  as  in  a  foundation,  on  account  of  its 
incompressible  nature,  of  adjusting  itself  to  a  uniform 
pressure  and  resisting  the  deformation  of  the  road-bed. 
If  the  small  pieces  composing  the  foundation  are 
cemented  together,  or  held  as  in  masses  of  angular 
fragments  by  the  interlocking  of  the  angles,  the  foun- 
dation may  act  more  or  less  as  a  whole,  causing  a  distri- 
bution of  the  load  over  a  considerable  area,  the  extent 
of  which  will  depend  upon  the  resistance  of  the  mass 
to  bending. 

The  bases  most  commonly  employed  for  pavements 
are  sand,  broken  stone,  and  concrete.  Foundations  of 
brick  and  wood  are  also  sometimes  employed  for  pave- 
ments of  the  same  materials. 


214       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.   56.     BASES  OF  GRAVEL  AND  BROKEN  STONE. 

For  light  service  on  a  good  road-bed  a  satisfactory 
foundation  may  frequently  be  constructed  of  gravel 
or  broken  stone  at  much  less  expense  than  would  be 
required  for  a  concrete  base.  A  foundation  of  this 
character  should  be  constructed  in  about  the  same 
manner  as  a  broken-stone  road,  the  material  being 
spread  over  the  road-bed  and  thoroughly  rolled  to 
the  required  form.  The  stone  or  gravel  employed 
should  contain  sufficient  small  material  to  fill  the 
voids  in  the  aggregate,  or  a  binding  material  may  be 
added  to  aid  in  compacting  the  foundation,  and  to 
close  the  interstices  so  as  to  prevent  any  settling  of 
the  material  which  is  used  to  support  the  paving 
surface. 

These  foundations  are  sometimes  used  under  brick 
pavements  of  light  traffic  with  good  results.  It  is 
important,  in  the  construction  of  brick  pavements  in 
this  manner,  that  the  interstices  in  the  base  be  thor- 
oughly filled,  as  otherwise  the  sand  cushion  under  the 
bricks  may  gradually  settle  into  the  foundation. 

A  foundation  of  this  kind  can  never  have  the  strength 
and  permanence  of  a  concrete  base,  and,  while  they 
may  give  good  results  when  well  constructed  under 
proper  conditions,  they  have  frequently  been  used 
where  a  small  additional  expense  for  concrete  would 
have  been  much  more  economical  in  the  end. 

ART.  57.     CONCRETE  BASES. 

The  best  base  for  general  use  under  pavements  is 
without  doubt  that  formed  of  hydraulic  cement  con- 
crete, A  bed  of  concrete  made  of  good  hydraulic 


FOUNDATIONS   FOR   PAVEMENTS.  215 

cement,  well  rammed  and  allowed  to  set  and  harden, 
becomes  a  practically  monolithic  structure,  nearly  im- 
pervious to  water  and  possessing  a  high  degree  of 
strength  against  crushing. 

The  concrete  is  formed  of  a  mixture  of  cement, 
sand,  and  broken  stone  or  gravel.  The  proportions 
vary  for  different  work  and  with  the  character  of  the 
materials.  With  good  Portland  cement  the  most 
common  proportions  for  ordinary  work  are  about  one 
part  cement,  3  parts  sand,  and  5  to  7  parts  broken 
stone.  With  the  various  natural  cements  the  pro- 
portions vary  somewhat,  but  are  usually  about  I  part 
cement,  2  parts  sand,  and  4  or  5  parts  of  stone  or 
gravel. 

Natural  cement  is  often  employed  for  this  pur- 
pose as  being  cheaper  and  possessing  ample  strength 
for  the  work,  and  concrete  of  the  ordinary  propor- 
tions with  natural  cement  is  to  be  preferred  to  that 
made  with  meager  proportions  of  Portland  cement 
giving  about  the  same  strength  and  cost.  Proper  tests 
should  always  be  imposed  for  the  purpose  of  securing 
good  cement.* 

Sand  for  use  in  mortar  should  be  as  clean  and  as 
free  from  loam,  mud,  or  organic  matter  as  possible. 
In  general  the  presence  of  any  foreign  matter  is  to  be 
avoided.  Coarse  sand  is  usually  preferable  to  that 
which  is  very  fine,  provided  it  be  fine  enough  to  give  a 
smooth  mortar,  as  it  affords  better  strength.  The  use 
of  a  mixture  of  grains  of  various  sizes  is  usually  desirable 
as  giving  less  voids  to  be  filled  by  the  cement. 

The  aggregate  used  for  concrete  should  be  as  hard 

*  For  discussion  of  lests  of  cement  see  "Hydraulic  Cement,  its  Prop- 
erties, Testing  and  Use,"  by  F.  P<  Spalding.  John  Wiley  and  Sons, 
New  York. 


216       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

and  durable  as  possible,  and  that  of  angular  form  is 
preferable  to  rounded.  The  materials  should  be  uni- 
form in  quality.  When  gravel  is  used  which  varies  in 
quality,  it  should  be  blended  by  mixing  in  order  to 
obtain  uniform  strength  in  the  concrete.  The  best 
concrete  will  usually  be  made  from  the  stone  contain- 
ing the  smallest  percentage  of  voids,  provided  the 
material  be  uniform.  In  a  mass  of  ordinary  broken 
stone  the  voids  are  usually  from  40  per  cent  to  55  per 
cent  of  the  volume.  This  may  be  considerably  reduced 
by  careful  adjustment  of  the  sizes.  The  broken  stone  is 
commonly  limited  in  size  to  2  or  2 J  inches,  and  the 
whole  output  of  the  crusher  is  used,  with  the  dust 
screened  out.  The  quantity  of  sand  needed  is  such  as 
will  fill  the  voids  in  the  aggregate. 

In  preparing  the  concrete,  the  cement  and  sand 
should  first  be  thoroughly  mixed  while  dry,  then  the 
proper  quantity  of  water  be  added  all  at  once,  and 
the  mortar  be  vigorously  worked  with  hoe  or  shovel 
for  2  or  3  minutes,  until  it  comes  to  a  smooth  and  uni- 
form condition. 

The  quantity  of  water  should  be  such  as  under 
energetic  working  will  reduce  the  mortar  to  a  soft, 
plastic  condition,  and  should  be  determined  by  meas- 
ure. The  application  of  the  water  from  a  hose  during 
the  mixing  is  objectionable  on  account  of  the  diffi- 
culty of  regulating  the  quantity  to  produce  mortar  of 
proper  consistency, 

When  the  mixing  of  the  mortar  is  complete,  the 
stone  or  gravel  may  be  added,  and  the  whole  mass 
turned  several  times  with  shovels  until  the  mortar  is 
evenly  distributed  through  the  aggregate.  The  stone 
should  be  wet  by  sprinkling  before  it  is  mixed  with 
the  mortar,  in  order  to  clean  the  surfaces  of  dust  and 


FOUNDATIONS   FOR   PAVEMENTS.  217 

to  prevent  the  absorption  of  water  from  the  mortar 
before  it  sets. 

The  concrete,  when  ready,  is  placed  in  position  and 
tamped  to  surface.  For  this  use  it  is  preferable  that 
the  concrete  be  of  jelly-like  consistency,  such  that  it 
will  quake  under  light  ramming.  The  rammer  com- 
monly employed  consists  of  a  block  of  wood,  or  of  cast 
iron,  6  to  8  inches  square,  flat  on  the  bottom,  and 
weighing  20  to  30  pounds.  The  tamping  should  cause 
the  mortar  to  flush  to  the  surface. 

After  completion  the  foundation  should  be  allowed 
to  stand  several  days  before  the  pavement  is  placed 
upon  it, —  3  to  6  days  are  usually  required, —  in  order 
that  the  mortar  may  become  entirely  set.  During 
setting  the  concrete  should  be  protected  from  the 
drying  action  of  the  sun  and  wind,  and  should  be  kept 
damp  to  prevent  the  formation  of  drying  cracks. 

The  quantity  of  material  necessary  to  make  a  cubic 
yard  of  concrete  varies  with  the  density  of  the  broken 
stone.  For  materials  measured  loose,  to  make  a  cubic 
yard  of  I,  2,  4  concrete  will  require  i^  to  ij  barrels  of 
natural  cement,  T4a  to  T5o  cubic  yard  of  sand,  and  T8<y  to 
I  cubic  yard  of  broken  stone.  To  make  I  cubic  yard  of 
I,  3,  6  concrete  requires  T8o  to  I  barrel  of  Portland 
cement,  T4a  to  T5o  cubic  yard  of  sand,  and  T%  to  I  cubic 
yard  of  broken  stone. 

ART.  58.     BITUMINOUS  FOUNDATIONS. 

Foundations  of  bituminous  concrete  are  frequently 
used  under  asphalt  and  bitulithic  pavements,  and,  in 
some  instances,  under  other  surfaces.  These  founda- 
tions are  constructed  in  much  the  same  way  as  bitu- 
minous macadam  roads  (see  Chap.  VI).  In  some 


2l8        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS 

instances  the  bases  are  formed  of  concrete  composed 
•of  broken  stone  and  tar,  or  asphalt  cement  mixed  in 
the  same  manner  as  the  binder  course  for  an  asphalt 
pavement  (see  Art.  70),  and  rolling  or  tamping  the 
concrete  into  place* 

The  more  common  method  of  construction  is  by 
spreading  and  rolling  the  broken  stone,  4  or  6  inches 
thick,  as  for  a  macadam  road,  and  covering  the  surface 
with  a  coating  of  bituminous  cement.  Coal-tar  cement 
is  ordinarily  used  for  this  purpose,  or  a  mixture  of  coal- 
tar  and  asphalt. 

The  bituminous  foundation  is  commonly  employed 
in  the  construction  of  bitulithic  pavements  (see  Art.  73). 
The  advantage  claimed  for  it  is  that  it  permits  the  over- 
lying courses  to  bind  into  the  foundation  and  holds 
the  surface  layer  in  place.  Foundations  of  this  kind 
give  good  results  when  the  road-bed  is  firm,  so  that  it 
may  be  rolled  solid  and  is  not  likely  to  become  unstable. 
It  has  not,  however,  the  stability  of  hydraulic  cement 
concrete  and  should  not  be  used  where  strength  is 
needed  or  where  the  road-bed  is  composed  of  spongy 
clay  or  other  material  which  cannot  be  rolled  to  pro 
vide  a  solid  sub-foundation, 

ART.  59     MISCELLANEOUS  FOUNDATIONS. 

Brick  Foundations,  Foundations  of  brick  have  fre- 
quently been  used  under  brick  pavements.  The  pave 
ment  in  such  cases  consists  of  two  layers  of  brick,  with 
sand  between,  and  is  known  as  double-layer  pavement, 
These  foundations  are  usually  formed  by  placing  upon 
the  road-bed  a  layer  of  sand  or  gravel  3  or  4  inches  thick, 
which  is  rolled  thoroughly  to  a  uniform  surface,  and 
then  receives  a  layer  of  brick,  commonly  laid  flat  and 


FOUNDATIONS   FOR   PAVEMENTS.  219 

with  the  greatest  dimension  lengthwise  of  the  street, 
These  bricks  are.  laid  as  closely  as  possible  with 
broken  joints.  The  joints  are  rilled  with  sand  care- 
fully swept  in,  and  the  bricks  are  rammed  to  a  firm 
bearing. 

Upon  this  course  of  brick  is  placed  a  cushion  layer 
of  sand,  and  then  the  surface  layer.  The  bricks  of  the 
lower  layer  may  be  of  a  cheaper  grade  than  the  sur- 
face paving  brick,  as  they  are  not  required  to  resist  the 
attrition  of  travel. 

Care  must  be  used  to  thoroughly  fill  the  joints  in  the 
foundation  layer  of  brick  in  order  that  the  sand  in  the 
cushion  layer  may  not  work  downward  and  allow 
the  surface  bricks  to  settle. 

These  foundations  were  formerly  quite  extensively 
used  for  brick  pavements,  but  have  for  the  most  part 
been  superseded  by  concrete  or  macadam  bases.  They 
have,  in  many  instances,  given  good  results  in  use 
when  resting  upon  a  firm  road  bed,  but  lack  the 
strength  of  the  concrete  foundation  and  are  not  usually 
economical. 

Sand  and  Plank  Foundation.  Under  many  wood 
pavements,  and  sometimes  under  brick  surfaces,  foun- 
dations formed  of  sand  and  planks  have  been  used. 
These  foundations  differ  somewhat  in  construction  in 
various  localities,  but  are  essentially  a  bed  of  sand  or 
gravel,  upon  which  is  placed  a  layer  of  tarred  boards 
which  support  the  surface  layer. 

It  is  common  to  use  a  layer  of  sand  3  or  4  inches 
thick,  which  is  compacted  by  rolling;  after  which  the 
boards  are  laid  lengthwise  of  the  street  close  together, 
so  as  to  form  a  floor  upon  which  the  blocks  may  be 
set.  With  a  brick  surface  a  cushion  coat  of  sand  is 
used  under  the  surface  layer. 


220       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Sometimes  two  layers  of  one-inch  tarred  boards  are 
employed,  the  lower  being  laid  crosswise  of  the  street 
and  the  upper  lengthwise  of  it.  In  other  cases  the 
boards  of  a  single  thickness  are  nailed  to  scantling  laid 
across  the  street  and  bedded  in  the  sand.  The  boards 
must  in  all  cases  press  evenly  upon  the  layer  of  sand 
that  covers  the  road-bed. 

These  foundations  were  used  under  the  round  block- 
wood  pavement,  at  one  time  quite  extensively.  They 
are  employed  only  where  low  cost  of  construction  is 
necessary,  and  are  not  economical  when  a  durable  road- 
surface  is  to  be  constructed. 

Sand  Foundations.  Brick  pavements  have  frequently 
been  constructed  with  only  a  cushion  coat  of  sand  upon 
the  earth  road-bed.  In  some  instances,  where  the 
road-bed  is  firm  and  well  drained,  forming  a  natural 
foundation,  this  method  of  construction  has  been 
successful  under  light  traffic,  but  the  failures  have  been 
numerous,  and  it  is  only  under  exceptional  circum- 
stances that  such  construction  will  prove  economical. 
The  same  method  has  been  applied  to  wood-block  and 
stone-block  pavements,  stone  blocks  being  usually  set 
in  a  bed  of  sand  or  gravel  4  to  8  inches  deep. 

ART.  60.     CHOICE  OF  FOUNDATION. 

It  is  always  important  that  the  foundation  be  suffi- 
cient. The  yielding  of  the  base  of  the  pavement  means 
its  destruction. 

If  a  firm  and  durable  foundation  be  employed,  the 
surface  may  be  renewed  when  necessary  or  changed 
from  one  material  to  another  without  disturbing  the 
base,  but  if  the  base  be  weak  the  surface  will  be 
destroyed. 


FOUNDATIONS   FOR   PAVEMENTS.  221 

The  saving  of  expense  should  be  at  the  top  rather 
than  at  the  bottom  of  a  pavement. 

The  thickness  required  for  the  foundation  of  a  pave- 
ment depends  upon  the  nature  of  the  soil  upon  which 
it  is  to  rest,  and  upon  the  extent  and  weight  of  the 
travel  to  which  it  is  to  be  subjected. 

When  the  road-bed  is  of  a  retentive  material  and 
likely  to  become  wet  and  soft,  the  foundation  should 
possess  sufficient  strength  not  to  be  broken  through  at 
points  where  the  supporting  po\ver  of  the  road-bed  may 
be  destroyed  by  water.  It  must  also  be  able  to  resist 
the  action  of  frost  upon  the  soil  below.  In  such  cases 
8  or  9  inches  of  concrete  may  be  necessary.  Six  inches 
of  good  concrete,  however,  constitute  a  foundation  of 
considerable  strength,  and  it  is  only  under  severe  con- 
ditions, poor  support  and  heavy  traffic,  that  a  greater 
depth  is  necessary. 

Under  light  traffic  with  good  conditions,  a  less  depth 
may  be  sometimes  used;  4  inches  of  concrete  is  fre- 
quently employed  to  save  expense,  although  6  is  the 
more  common  depth.  A  depth  of  4  or  6  inches  of  well 
compacted  gravel  or  broken  stone  is  also  usually  .suffi- 
cient where  the  conditions  are  such  as  to  admit  of  the 
use  of  a  foundation  of  that  character. 

It  may  be  here  observed  that  no  definite  prescrip- 
tion for  any  pavement,  either  as  to  choice  of  founda- 
tion or  as  to  methods  of  construction,  can  fit  all  cases. 
What  is  most  successful  in  one  ^ase  is  quite  inappli- 
cable in  another,,  The  blind  following  of  particular 
rules  by  those  not  conversant  with  the  principles  upon 
which  they  are  based  has  been  the  cause  of  many  fail- 
ures. Judgment  must  always  be  used  in  weighing  the 
local  conditions  of  the  problem  in  hand. 


CHAPTER  VIII. 

BRICK  PAVEMENTS. 
ART.  61.     PAVING-BRICK. 

THE  requisites  for  a  good  paving-brick  are  that  it 
shall  be  hard,  tough,  and  impervious,  as  well  as  capable 
of  enduring  against  the  disintegrating  influences  of  the 
weather. 

The  bricks  in  most  common  use  are  made  from  fire- 
clay of  an  inferior  quality,  or  from  an  indurated  clay  or 
shale  of  somewhat  similar  composition. 

These  clays  consist  essentially  of  silicate  of  alumina, 
with  usually  small  percentages  of  lime,  magnesia,  iron, 
potash,  soda,  and  sometimes  other  elements.  The 
range  of  composition  for  clays  in  common  use  is 
approximately  as  follows: 

Percent 

Silica 60  to  75 

Alumina 1 0  to  25 

Iron  oxide 3  to    8 

Lime o  to    4 

Magnesia 0  to    3 

Potash 0.5  to    3 

Soda o  to    2 

In  a  few  cases  the  quantity  of  lime  is  greater,  vary- 
ing from  8  to  12  per  cent. 

When  the  clay  is  very  nearly  pure  silicate  of  alumina, 
it  is  capable  of  withstanding  a  high  degree  of  heat 
without  fusing,  and  is  known  as  fire-clay.  As  the  per- 


BRICK   PAVEMENTS.  223 

centages  of  other  ingredients  increase,  it  becomes  more 
fusible.  The  lime,  magnesia,  potash,  and  soda  act  as 
fluxing  agents,  and  the  readiness  with  which  the  clay 
can  be  melted  depends  upon  the  relative  quantities  of 
refractory  and  fluxing  materials  that  it  may  contain. 

Silica  in  excess  tends  to  make  the  brick  weak  and 
brittle,  while  too  great  quantity  of  alumina  causes  the 
brick  to  crack  and  warp  in  the  shrinking  which  occurs 
during  burning.  The  proper  adjustment  of  the  rela- 
tions between  these  elements  is  necessary  to  good 
results. 

The  quantity  of  lime  in  the  clay  is  an  important 
matter,  as  the  presence  of  lime  in  an  uncombined  state 
in  the  brick  may  be  productive  of  disintegration  when 
the  brick  is  exposed  to  the  weather.  A  large  percent- 
age of  lime  in  a  clay  is  therefore  to  be  regarded  with 
suspicion,  although  not  necessarily  as  cause  for  con- 
demnation, as  its  effect  depends  upon  the  state  of  com- 
bination of  the  various  ingredients  of  the  brick.  Mag- 
nesia probably  acts  in  much  the  same  manner  as  lime. 
Potash  and  soda  are  considered  desirable  elements  in 
quantities  to  properly  flux  the  clay  in  burning. 

The  fineness  of  a  clay  is  also  a  matter  of  importance, 
both  because  a  fine  clay  will  fuse  at  a  lower  tempera- 
ture than  a  coarse  one,  and  because  fineness  is  neces- 
sary to  the  production  of  even  and  close  grained  brick, 
and  therefore  conduces  to  make  them  tough  and 
impervious. 

To  produce  a  good  paving-brick,  a  clay  is  required 
which  will  vitrify  at  a  high  heat.  A  very  refractory 
clay  will  make  a  porous  brick,  wrhile  if  it  melts  at  too 
low  a  temperature  it  cannot  be  burned  sufficiently  to 
become  hard  and  tough, 

The  methods  of  manufacturing  paving- brick  vary  in 


224       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

different  localities  according  to  the  character  of  the 
material  to  be  worked.  They  are  quite  similar  to 
those  in  use  for  common  brick,  only  more  thoroughly 
executed. 

The  clay  is  commonly  reduced  to  a  fine  powder, 
tempered  with  water  and  passed  through  a  machine 
that  molds  the  bricks,  which  are  then  dried  and  after- 
ward burned.  Repressed  bricks  are  those  which  are 
compressed  in  a  mold  after  coming  from  the  brick 
machine  and  before  drying. 

The  process  of  burning  occupies  usually  from  10  to 
15  days-. 

The  heat  is  at  first  slowly  applied  to  expel  the  water, 
then  raised  to  a  high  temperature  for  several  days, 
after  which  the  bricks  are  very  slowly  cooled. 

There  is  considerable  difference  of  opinion  among 
engineers  and  manufacturers  as  to  the  exact  amount  of 
burning  necessary.  It  is  usually  stated  that  the  brick 
should  be  burned  to  the  point  of  vitrification,  but  not 
completely  vitrified.  The  burning  must  be  thoroughly 
done  to  produce  a  strong  and  impervious  brick,  but 
there  is  undoubtedly  a  point  beyond  which,  for  some 
brick,  further  burning  causes  brittleness.  Very  gradual 
cooling  is  also  necessary  in  order  to  toughen  the  brick. 
Smoothness  and  uniformity  of  texture  in  a  paving 
brick  is  an  important  consideration  as  affecting  its  re- 
sistance both  to  crushing  and  to  abrasion.  The  broken 
surface  of  the  brick  should  present  a  uniform  appear- 
ance both  in  texture  and  in  color. 

All  the  bricks  used  in  the  same  pavement  should 
also  be  of  the  same  degree  of  hardness  and  toughness 
in  order  that  the  pavement  may  wear  evenly,  and  to 
this  end  careful  inspection  should  always  be  given  to 
the  bricks  proposed  for  use,  and  all  of  those  which  are 


BRICK  PAVEMENTS.  225 

defective,  soft  from  imperfect  burning,  brittle  from 
overburning  or  quick  cooling,  cracked  or  distorted  by 
unequal  shrinkage,  should  be  rejected.  An  examina- 
tion of  the  color  and  size  of  the  bricks  may  frequently 
be  useful  in  determining  for  any  particular  material 
whether  individual  bricks  have  received  the  proper 
degree  of  burning,  after  the  engineer  has  become 
familiar  with  the  make  of  brick  under  examination. 
The  amount  of  shrinkage  in  burning  is  often  a  quite 
reliable  index  of  the  degree  of  burning  to  which  the 
material  has  been  subjected,  and  specifying  within 
somewhat  narrow  limits  the  variation  in  size  of  bricks 
to  be  used  together  may  often  conduce  to  greater 
uniformity  in  the  material  employed.  Some  makes  of 
brick  vary  quite  appreciably-  in  size  for  small  differ- 
ences in  extent  of'  burning,  and  without  materially 
affecting  the  value  of  the  product,  but  it  is  desirable 
to  sort  them  closely  and  use  those  of  each  size  by 
themselves. 

The  mistake  is  frequently  made  of  placing  too  high 
value  upon  the  element  of  hardness,  which  when  car- 
ried to  an  extreme  is  sometimes  attained  at  the  ex- 
pense of  toughness,  the  brick  becoming  brittle  and 
easily  shattered.  The  author  (under  his  guaranty 
on  a  pavement)  has,  on  one  occasion,  been  obliged  to 
replace  a  small  number  of  hard  bricks,  which  at  the 
time  of  laying  were  supposed  to  be  among  the  best 
of  the  lot,  on  account  of  their  becoming  shattered  under 
traffic,  while  somewhat  softer  brick  from  the  same  kilns, 
the  use  of  which  was  questioned  by  the  inspector, 
proved  quite  satisfactory  in  the  same  work. 

The  sizes  of  paving-bricks  vary  considerably  as 
made  by  different  manufacturers,  the  most  common 
sizes  approximating  to  those  of  building  brick,  varying 


226       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

from  about  2\  to  2|-  inches  in  width,  8  to  8J  inches 
long,  and  4  inches  deep,  about  56  to  63  bricks  being 
required  for  a  square  yard  of  pavement.  A  few 
makers  also  produce  a  brick,  of  about  the  same  size 
but  only  3^  inches  deep.  A  larger  size,  usually  about 
3X9X4  inches,  and  known  as  a  paying-block  to 
distinguish  it  from  the  smaller  brick,  is  quite  largely 
employed.  These  usually  require  from  43  to  47 
blocks  per  square  yard. 

Larger  blocks  have  been  tried,  but  have  not  come 
into  general  use,  while  some  manufacturers  make 
smaller  sizes,  requiring  70  to  75  bricks  to  the  square 
yard  of  pavement. 

Opinions  differ  as  to  the  best  sizes  for  use  in  pave- 
ments, some  engineers  specifying  the  smaller  bricks, 
others  the  larger  blocks.  A  good  pavement  can 
be  built  of  either  if  proper  attention  be  given  to 
selecting  the  material.  The  sizes  preferred  by  the 
various  manufacturers  depend  largely  upon  the 
character  of  the  clay  or  shale  with  which  they  have  to 
deal.  With  some  materials  the  size  is  limited  by  the 
distortion  of  large  blocks  in  burning,  and  the  smaller 
bricks  are  preferalle;  with  others,  larger  blocks  may 
be  made  at  less  cost  in  proportion  to  area  of  pavement, 
and  perhaps  with  better  and  smoother  work  resulting. 
There  seems  to  be  no  necessity  for  any  increase  in  the 
usual  depth  of  4  inches  as  is  sometimes  proposed,  and 
it  may  be  possible  that  the  adoption  of  the  depth,  3^ 
inches,  now  frequently  used  may  in  many  instances 
somewhat  lessen  the  cost  of  the  pavement  without 
affecting  its  length  of  service. 

Much  difference  of  opinion  has  been  developed 
among  engineers  as  to  the  advisability  of  rounding  the 
corners  of  the  brick,  some  requiring  that  the  blocks  be 


BRICK  PAVEMENTS.  227 

repressed  with  corners  rounded  to  a  radius  of  J  01  f  of 
an  inch,  while  others  specify  square-edge  brick,  and 
in  some  instances  that  they  shall  not  be  repressed. 
On  the  one  hand,  it  is  maintained  that  in  service  the 
sharp  corners  will  soon  be  knocked  off  and  worn 
down  more  roughty  and  unevenly  than  if  originally 
rounded,  while,  on  the  other  hand,  it  is  claimed 
that  if  a  rigid  filler  like  Portland  cement  be  used,  the 
joint  may  be  filled  level  with  the  surface  of  the  brick, 
and  be  much  less  likely  to  chip  out  than  if  the  joint 
be  widened  at  the  top  so  as  to  cause  the  filler  to  pre- 
sent a  thin  edge  at  the  sides.  Both  contentions  seem 
reasonable  under  certain  conditions,  and  the  method 
of  construction  and  character  of  filler  used  will  ordi- 
narily determine  the  proper  form  for  the  brick. 

The  desirability  of  repressing  the  brick  is  also  a 
much  discussed  question,  it  being  argued  by  some  that 
the  repressing  of  the  material  forms  a  more  dense  and 
compact  block  and  increases  its  probable  wear  in  use, 
and  by  others  that  the  pressure  applied  to  the  material 
after  it  comes  from  the  brick-machine  disturbs  the 
structure  and  injures  the  fiber  of  the  brick,  often 
forming  laminations  which  are  elements  of  weakness. 
With  some  materials  this  last  contention  seems  to 
have  some  basis  in  fact,  but  in  other  and  probably 
most  materials  no  such  condition  can  be  found  on 
examination  of  the  structure  of  the  brick.  The  views 
of  individual  manufacturers  upon  the  question  seem  to 
depend  mainly  upon  the  kind  of  material  they  have 
to  work  with,  and  it  would  be  difficult  from  existing 
data  to  say  that  either  method  necessarily  gives  the 
best  results.  Possibly  those  materials  which  approach 
most  nearly  to  actual  vitrification  and  are  subject  to 
considerable  shrinkage  during  the  burning  are  but 


228       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

little  affected  in  final  density  by  the  compression  of 
the  block  before  burning.  It  should  be  observed  in 
this  connection  that  whatever  may  be  the  value  of 
repressing  as  to  its  effect  upon  the.  quality  of  the  brick 
and  wear  of  the  pavement,  it  undoubtedly  has  the 
effect  with  some  kinds  of  brick  of  giving  a  smoother 
and  better  surface  to  the  pavement  by  producing  a 
more  regular  and  uniformly  shaped  brick. 

In  order  to  give  sufficient  space  between  the  bricks 
for  the  joint-filling,  some  manufacturers  make  re- 
pressed bricks  with  lugs  on  one  side  to  hold  them  a 
given  distance  apart  when  laid  in  the  pavement.  The 
wisdom  of  this  under  ordinary  circumstances  seems 
doubtful,  as  small  joint-space  is  usually  desirable,  and 
experience  shows  that  bricks  laid  close,  even  if  care- 
fully driven  up,  will  usually  give  plenty  of  space  for 
filling.  Spacing-lugs  are  seldom  required  in  specifica- 
tions, but  engineers  have  sometimes  required  their  use 
for  work  on  steep  grades  with  the  idea  of  giving 
better  foothold  to  horses  than  the  thin  joints  would 
afford.  There  may  be  some  advantage  in  this,  and 
some  pavements  so  constructed  on  grades  of  above 
eight  per  cent  have  given  satisfactory  results,  the  lugs 
usually  projecting  about  half  an  inch  from  the  face  of 
the  brick,  but  in  the  author's  own  experience  he  has 
been  unable  to  notice  any  difference  in  the  use  of  the 
pavement  with  or  without  the  wide  joints,  all  being 
laid  with  bevel-edge  brick. 

Repressed  bricks  and  blocks  are  frequently  made 
with  a  groove  or  two,  extending  lengthwise  of  the 
brick  on  each  side  and  sometimes  across  the  ends,  for 
the  purpose  of  keying  the  blocks  together  when  filled 
with  the  joint-filler .  This  may  be  an  aid  to  rigid 
construction,  though  not  necessary  to  good  work. 


BRICK   PAVEMENTS.  229 

It  is  usually  limited  to  the  larger  blocks,  which  fre- 
quently have  thin  lugs,  as  well  as  the  grooves,  and 
are  known  as  groove-  and  lug-blocks.  They  are  well 
calculated  to  give  a  very  firm  construction. 

ART,  62.     TESTS  FOR  PAVIXG-BRICK. 

To  determine  the  probable  durability  of  brick 
designed  for  use  in  paving,  mechanical  tests  may  be 
applied  which  will  show  the  relative  rank  of  different 
samples  in  their  most  important  characteristics.  It  is, 
however,  a  matter  of  considerable  difficulty  to  set  a 
standard  to  which  the  brick  should  be  required  to  con- 
form, or  to  determine,  from  the  behavior  of  the  bricks 
under  test,  the  relative  value  of  various  samples  which 
it  may  be  desired  to  compare. 

The  tests  ordinarily  proposed  or  used  for  this  purpose 
are  those  of  crushing  strength,  transverse  strength, 
abrasion  and  impact,  absorption,  and  specific  gravity. 
The  relative  importance  of  these  tests  and  the  weight 
which  should  be  given  to  their  results  is  a  matter 
concerning  which  considerable  difference  of  opinion  has 
been  developed  amongst  engineers,  and  practice  va- 
ries considerably.  The  National  Brick  Manufacturers' 
Association  have  considered  the  matter,  and  in  1895 
appointed  a  committee  which  in  1897  reported  a  set 
of  rules  for  making  the  tests,  with  resolutions  expressing 
their  views  as  to  the  relative  importance  and  reliability 
of  each.  These  rules,  which  were  somewhat  modified 
in  1900,  are  very  commonly  followed  and  furnish  a 
standard  method  of  testing. 


230        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 
CRUSHING   TEST. 

The  recommendations  of  the  commission  for  con- 
ducting this  test  are  as  follows: 

"I.  The  crushing  test  should  be  made  on  half- 
bricks,  loaded  edgewise,  or  as  the}^  are  laid  in  the 
street.  If  the  machine  used  is  unable  to  crush  a  full 
half-brick,  the  area  may  be  reduced  by  chipping  off, 
keeping  the  form  of  the  piece  to  be  tested  as  nearly 
prismatic  as  possible.  A  machine  of  at  least  100,000 
pounds  capacity  should  be  used,  and  the  specimen 
should  not  be  reduced  below  4  square  inches  of  area 
in  cross-section  at  right  angles  to  direction  of  load. 

"II.  The  upper  and  lower  surfaces  should  prefer- 
ably be  ground  to  true  and  parallel  planes.  If  this  is 
not  done,  they  should  be  bedded  in  plaster  of  Paris 
while  in  the  testing-machine,  which  should  be  allowed 
to  harden  ten  minutes  under  the  weight  of  the  crush- 
ing planes  only  before  the  load  is  applied. 

"III.  The  load  should  be  applied  at  a  uniform  rate 
of  increase  to  the  point  of  rupture. 

"IV.  Not  less  than  an  average  obtained  from  5 
tests,  on  5  different  bricks,  shall  constitute  a  stand- 
ard test/' 

The  result  of  a  compressive  test  of  stone  or  brick 
depends  very  largely  upon  how  it  is  made,  and  the 
results  of  tests  are  only  properly  comparable  with 
others  made  in  the  same  manner  and  with  equal  care. 
The  use  of  plaster  beds  as  suggested  above,  it  is 
thought,  conduces  greatly  to  regularity  of  result  in  the 
work  of  different  men,  as  it  tends  to  reduce  the  effect 
of  differences  in  the  accuracy  of  dressing  the  surfaces 
of  contact.  The  size  of  the  test -piece  is  also  impor- 
tant, the  strength  usually  increasing  as  the  size  in- 


BRICK  PAVEMENTS.  23! 

creases.  Small  pieces,  i£-  or  2-inch  cubes,  are  often 
employed  because  of  the  large  force  necessary  to  crush 
a  whole  or  half  brick,  although  where  machinery  exists 
capable  of  doing  it  the  larger  tests  entail  much  less 
work  in  preparing  specimens  and  also  yield  much 
more  satisfactory  results.  Where  small  specimens  are 
used  it  is  to  be  observed  that  the  unit  strength  will 
not  be  the  same  as  for  larger  ones,  and  must  be  judged 
by  a  different  standard.  In  the  preparation  of  speci- 
mens it  is  better,  when  possible,  to  saw  than  to  break 
them  by  chipping,  in  order  not  to  injure  the  block  by 
the  shock  of  the  blows. 

The  commission  in  their  discussion  concluded  that 
no  connection  has  been  shown  between  high  strength 
and  the  qualities  necessary  for  a  good  paving  material, 
and  adopted  the  following  resolution: 

"WHEREAS,  From  the  experimental  work  done  so 
far  by  this  commission,  or  by  others  so  far  as  is  known 
to  us,  in  the  application  of  the  cross-breaking  and 
crushing  tests  to  paving-bricks,  it  is  not  possible  to 
show  any  close  relationship  between  the  qualities 
necessary  for  a  good  paving  material  and  high  struc- 
tural strength  as  indicated  by  either  of  these  tests, 

"Resolved,  That  for  this  reason  the  commission  rec- 
ommends that  these  tests  shall  be  considered  as  purely 
optional  in  the  examination  of  paving  material,  and 
not  necessary  as  a  proof  of  excellence/* 

It  is  to  be  observed  that  the  actual  crushing  strength 
of  a  brick  is  not  a  matter  of  special  importance  in  so 
far  as  any  danger  of  the  crushing  of  the  material  in  the 
pavement  is  concerned,  as  no  stress  can  there  come 
upon  it  under  ordinary  circumstances  which  would 
endanger  even  a  very  weak  specimen  from  direct 
crushing.  It  is  thought,  however,  that  to  some  extent 


232       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  value  of  the  brick  is  indicated  by  its  resistance  to 
crushing,  coupled,  of  course,  with  a  proper  examina- 
tion of  its  other  necessary  attributes.  A  brick  which 
possesses  a  high  crushing  strength  is  not  necessarily 
a  good  paving-brick,  as  it  may  at  the  same  time  be 
brittle  or  of  such  composition  as  to _  easily  disintegrate 
under  the  action  of  the  weather;  but  one  that  yields 
to  a  low  crushing  strength  is  usually  weak  in  wearing 
qualities  and  not  fit  for  the  purpose.  For  this  reason 
this  test  is  commonly  included  in  specifications  pre- 
•  scribing  tests,  although  it  is  recognized  that  the  rela- 
tive wearing  qualities  of  various  makes  of  brick  can- 
not be  graded  by  its  results.  A  good  paving-brick, 
in  the  form  of  a  2-inch  cube,  will  usually  show  a  re- 
sistance to  crushing  of  not  less  than  10,000  pounds 
per  square  inch.  Much  higher  values  are  sometimes 
used  in  specifications,  but  their  advantage  is  at  least 
doubtful. 

TRANSVERSE    TEST. 

The  transverse  strength  is  tested  by  supporting  the 
brick  upon  two  knife-edges  near  its  ends  and  bringing 
a  load  through  a  third  knife-edge  upon  the  middle  of 
the  brick.  The  test  may  be  made  upon  any  ordinary 
testing-machine  by  providing  the  necessary  knife- 
edges,  but,  like  the  compression  test,  requires  care  in 
manipulation  to  get  good  results.  It  is  specially  im- 
portant that  the  brick  have  a  perfectly  even  bearing 
upon  the  supports  before  the  application  of  the  load, 
in  order  that  it  may  not  be  subjected  to  a  twist  under 
the  load.  The  method  adopted  by  the  commission 
for  this  test  is  as  follows : 

I.  Support  the  brick  on  edge,  or  as  laid  in  the 
pavement,  on  hardened  steel  knife-edges  rounded 


BRICK  PAVEMENTS.  233 

longitudinally  to  a  radius  of  12  inches  and  trans- 
versely to  a  radius  of  one-eighth  inch,  and  bolted  in 
position  so  as  to  secure  a  span  of  six  inches. 

II.  Apply  the  load  to  the  middle  of  the  top  face 
through   a  hardened   steel   knife-edge,   straight   longi- 
tudinally and  rounded  transversely  to  a  radius  of  one- 
sixteenth  inch. 

III.  Apply  the  load  at  a  uniform  rate  of  increase 
till  fracture  ensues. 

IV.  Compute  the  modulus  of  rupture  by  the  for- 
mula 

3    wl 


in  which  /  =  modulus  of  rupture  in  pounds  per  square 

inch; 
w   --=  total  breaking  load  in  pounds; 

/  =  length  of  span  in  inches  =  6; 
[  b  =  breadth  of  brick  in  inches; 

d  =  depth  of  brick  in  inches. 

V.  Samples  for  test  must  be  free  from  all  visible 
irregularities  of  surface  or  deformities  of  shape,  and 
their  upper  and  lower  faces  must  be  practically  parallel. 

VI.  Not  less  than  10  brick  shall  be  broken  and  the 
average  of  all  be  taken  for  a  standard  test. 

The  commission  included  this  test  with  the  crush- 
ing test  in  the  recommendation  that  the  test  was  to 
be  considered  optional  and  "not  necessary  as  a  proof 
of  excellence.  "  This  test  is  easier  to  conduct  satis- 
factorily, and  probably  gives,  in  general,  a  more  reli- 
able indication  of  the  value  of  the  material  than  the 
crushing  test.  It  calls  into  play  the  tensile  as  well 
as  compressive  strength  of  the  brick.  The  interior 
structure  is  shown  by  the  break,  and  an  opportunity  is 


234       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

given  to  judge  of  the  uniformity  and  homogeneous 
character  of  the  material. 

The  fracture  of  a  tough  and  homogeneous  speci- 
men under  a  transverse  load  should  be  a  clean  break 
through  the  middle  of  the  brick,  and  a  close  observa- 
tion of  the  breaks  may  frequently  be  of  considerable 
assistance  in  forming  an  idea  of  these  qualities,  al- 
though they  may  not  be  directly  represented  by  the 
load  required  to  break  the  specimen.  The  shattering 
of  the  brick  in  breaking,  or  an  irregular  break  extend- 
ing from  the  point  of  application  of  the  load  to  one 
of  the  points  of  support,  usually  indicates  brittleness 
of  the  material. 

The  modulus  of  rupture  of  good  paving-bricks 
commonly  ranges  between  2000  and  3000  pounds  per 
square  inch,  sometimes  reaching  3500  or  even  4000 
pounds.  It  is  usually  somewhat  greater  for  brick  laid 
flat  than  for  brick  on  edge. 

ABRASION    TEST. 

In  the  convention  of  1897  the  Brick  Manufacturers' 
Association  adopted  a  method  for  this  test  consisting 
in  rattling  a  given  charge  of  bricks  in  a  cylinder  rotat- 
ing about  its  axis,  which  is  horizontal,  and  depending 
for  its  result  upon  the  impact  and  abrasion  of  the 
bricks  upon  each  other.  In  1900,  however,  after 
more  fully  considering  the  matter,  the  test  was  modi- 
fied, and  a  smaller  charge  of  bricks,  with  the  addition 
of  a  charge  of  cast-iron  shot,  was  recommended  as 
more  nearly  representing  the  conditions  of  practice 
and  giving  results  more  in  accord  with  experience. 

The  method  finally  recommended  by  the  Associa- 
tion is  as  follows: 


BRICK   PAVEMENTS.  235 

"/.  Dimensions  of  the  Machine,  The  standard 
machine  shall  be  28  inches  in  diameter  and  20  inches 
in  length,  measured  inside  the  rattling-chamber. 

"Other  machines  may  be  used  varying  in  diameter 
between  26  and  30  inches,  and  in  length  from  18  to 
24  inches;  but  if  this  is  done,  a  record  of  it  must  be 
attached  to  official  report.  Long  rattlers  may  be  cut 
up  into  sections  of  suitable  length  by  the  insertion  of 
an  iron  diaphragm  at  the  proper  point. 

"  //.  Construction  of  the  Machine.  The  barrel  shall 
be  supported  on  trunnions  at  either  end;  in  no 
case  shall  a  shaft  pass  through  the  rattling-chamber. 
The  cross-section  of  the  barrel  shall  be  a  regular 
polygon  having  14  sides.  The  heads  and  staves 
shall  be  composed  of  gray  cast  iron,  not  chilled  or 
case-hardened.  There  shall  be  a  space  of  one-fourth 
of  an  inch  between  the  staves  for  the  escape  of  dust 
and  small  pieces  of  waste.  Other  machines  may  be 
used  having  from  12  to  16  staves,  with  openings 
from  one-eighth  to  three-eighths  of  an  inch  between 
staves;  but  if  this  is  done,  a  record  of  it  must  be 
attached  to  the  official  report  of  the  test. 

"III.  Composition  of  the  Charge.  All  tests  must 
be  executed  on  charges  containing  but  one  make  of 
brick  or  block  at  a  time.  The  charge  shall  consist  of 
9  paving-blocks  or  12  paving-bricks,  together  with 
300  pounds  of  shot  made  of  ordinary  machinery  cast 
iron.  This  shot  shall  be  of  two  sizes,  as  described 
below,  and  the  shot -charge  shall  be  composed  of  one- 
fourth  (75  pounds)  of  the  larger  size  and  three-fourths 
(225  pounds)  of  the  smaller  size, 

"IV.  Size  of  the  Shot.  The  larger  size  shall  weigh 
about  7^  pounds  and  be  about  2\  inches  square  and 
4J  inches  long,  with  slightly  rounded  edges.  The 


236       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

smaller  size  shall  be  cubes  of  I  i  inches  on  a  side,  with 
rounded  edges.  The  individual  shot  shall  be  replaced 
by  new  ones  when  they  have  lost  one-tenth  of  their 
original  weight. 

"  V,  Revolutions  of  the  Charge.  The  number  of 
revolutions  of  a  standard  test  shall  be  1800,  and  the 
speed  of  rotation  shall  not  fall  below  28  nor  exceed 
30  per  minute.  The  belt -power  shall  be  sufficient  to 
rotate  the  rattler  at  the  same  speed  whether  charged 
or  empty. 

"  VI.  Condition  oj  the  Charge.  The  bricks  com- 
posing the  charge  shall  be  dry  and  clean  and  as 
nearly  as  may  be  possible  in  the  condition  in  which 
they  were  drawn  from  the  kiln. 

"VII.  Calculation  of  the  Result.  The  loss  shall 
be  calculated  in  the  per  cents  of  the  weight  of  the 
dry  brick  composing  the  charge,  and  no  result  shall 
be  considered  as  official  unless  it  is  the  average  of  two 
distinct  and  complete  tests  made  on  separate  charges 
of  brick/' 

The  commission  regard  this  as  the  most  important 
test  to  be  applied  to  paving-brick,  and  in  fact  it  is 
the  only  one  having  their  indorsement.  It  seems 
reasonable  to  suppose  that  this  test  gives  more  nearly 
than  the  others  a  determination  of  the  value  of  the 
brick  for  use  in  a  pavement. 

It  is  quite  true  that  the  action  to  which  the  brick 
is  subjected  in  a  test  of  this  character  is  different 
from  the  wear  to  which  it  is  subjected  when  firmly 
held  in  the  pavement,  but  the  qualities  necessary  to 
resist  wear  in  the  two  cases  are  very  similar.  We 
may  form  an  idea  of  whether  a  material  is  suitable  for 
the  proposed  use  from  such  experiments,  although  no 
definite  idea  of  the  amount  of  wear  that  it  will  en- 


BRICK   PAVEMENTS.  237 

dure  can  be  obtained  from  them.  It  should  also  be 
pointed  out  that  the  method  of  estimating  the  loss  of 
the  brick,  from  abrasion  tests  made  in  this  manner,  as 
percentages  of  the  total  weight  of  brick,  can,  in  the 
comparison  of  different  bricks,  only  give  correct  re- 
sults when  the  bricks  compared  are  of  the  same  size 
and  shape.  A  brick  with  rounded  edges  evidently 
could  not  properh7  be  compared  with  one  with  sharp 
edges  by  this  method,  and  some  engineers  have  divided 
the  test  into  two  periods  for  the  purpose  of  separat- 
ing the  knocking  off  of  the  corners  and  preliminary 
rounding  of  the  brick  from  the  later  abrasion  upon 
the  rounded  surfaces  which  would  be  more  nearly 
comparable  for  different  specimens.  If,  in  the  test, 
the  loss  in  the  rattler  during  the  first  half-hour  be 
separated  from  that  during  the  second  half-hour, 
the  latter  will  be  found  to  be  much  less  affected  by  the 
form  of  the  brick. 

In  comparing  bricks  of  different  sizes  it  should  be 
noted  that  a  small  brick  presents  more  surface  for 
abrasion  than  a  large  one  in  proportion  to  its  volume, 
and  the  results  of  such  comparisons  would  be  con- 
siderably modified  in  some  instances  if  the  results  be 
stated  in  terms  of  exposed  surface  instead  of  percent- 
age of  volume.  With  square-edged  brick  during  the 
early  period  of  the  test,  when  corners  are  being 
chipped  off,  the  loss  is  probably  more  nearly  propor- 
tional to  length  of  edges  than  to  surface  or  volume, 
which  would  be  still  more  to  the  disadvantage  of  the 
small  brick.  Care  is  therefore  necessary  in  drawing 
conclusions  from  such  tests  concerning  the  relative 
values  of  different  materials  that  all  the  conditions 
which  may  affect  such  conclusions  be  fully  under- 
stood. 


238      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

New  Abrasion  Test.  The  abrasion  test  as  above 
described,  after  being  in  use  for  a  number  of  years,  was 
found  somewhat  unsatisfactory,  on  account  of  the  varia- 
tion in  results  obtained  in  different  laboratories,  and  the 
National  Paving  Brick  Manufacturers'  Association,  in 
1911,  recommended  the  adoption  of  a  new  machine, 
together  with  specifications  for  its  use.  These  recommenda- 
tions are  as  follows: 

THE  RATTLER. 

"  The  machine  shall  be  of  good  mechanical  construction, 
self-contained,  and  shall  conform  to  the  following  details, 
of  material  and  dimensions,  and  shall  consist  of  barrel, 
frame  and  driving  mechanism  as  herein  described. 

THE  BARREL. 

"  The  barrel  of  the  machine  shall  be  made  up  of  the 
heads,  headliners  and  staves. 

"  The  heads  shall  be  cast  with  trunnions  in  one  piece. 
The  trunnion  bearings  shall  not  be  less  than  two  and  one- 
half  (2!)  inches  in  diameter  or  less  than  six  (6)  inches  in 
length. 

"  The  heads  shall  not  be  less  than  three-fourths  (J) 
inch  thick  nor  more  than  seven -eighths  (f )  inch.  In  out- 
line they  shall  be  a  regular  fourteen-sided  (14)  polygon 
inscribed  in  a  circle  twenty-eight  and  three-eighths  (23!) 
inches  in  diameter.  The  heads  shall  be  provided  with 
flanges  not  less  than  three-fourths  (f)  inch  thick  and 
extending  outward  two  and  one-half  (2%)  inches  from 
he  inside  face  of  head  to  afford  a  means  of  fastening  the 
staves.  The  flanges  shall  be  slotted  on  the  outer  edge, 
so  as  to  provide  for  two  (2)  three-fourths  (f)  inch  bolts 
at  each  end  of  each  stave,  said  slots  to  be  thirteen-sixteenths 


BRICK  PAVEMENTS.  239 

inch  wide  and  two  and  three-fourths  (2f)  inches 
center  to  center.  Under  each  section  of  the  flanges  there 
shall  be  a  brace  three-eighths  (f )  inch  thick  and  extending 
down  the  outside  of  the  head  not  less  than  two  (2)  inches. 
Each  slot  shall  be  provided  with  recess  for  bolt  head,  which 
shall  act  to  prevent  the  turning  of  the  same.  There  shall 
be  for  each  head  a  cast-iron  headliner  one  (i)  inch  in 
thickness  and  conforming  to  the  outline  of  the  head, 
but  inscribed  in  a  circle  twenty-eight  and  one-eighth  (28^) 
inches  in  diameter.  This  liner  or  wear  plate  shall  be 
fastened  to  the  head  by  seven  (7)  five-eighths  (3-)  inch 
cap  screws,  through  the  head  from  the  outside.  These 
wear  plates,  whenever  they  become  worn  down  one-half 
(J)  inch  below  their  initial  surface  level,  at  any  point  of 
their  surface,  must  be  replaced  with  new.  The  metal 
of  which  these  wear  plates  are  to  be  composed  shall  be 
what  is  known  as  hard  machinery  iron,  and  must  con- 
tain not  less  than  one  (i)  per  cent  of  combined  carbon. 
The  faces  of  the  polygon  must  be  smooth  and  give  uni- 
form bearing  for  the  staves.  To  secure  the  desired 
uniform  bearing  the  faces  of  the  head  may  be  ground 
or  machined. 

THE  STAVES. 

"  The  staves  shall  be  made  of  six  (6)  inch  medium 
steel  structural  channels  twenty-seven  and  one-fourth 
(27^)  inches  long  and  weighing  fifteen  and  five- tenths 
(15.5)  pounds  per  lineal  foot. 

;'  The  channels  shall  be  drilled  with  holes  thirteen- 
sixteenths  (Jf )  inch  in  diameter,  two  (2)  in  each  end, 
for  bolts  to  fasten  same  to  head,  the  center  line  of  the 
holes  being  one  (i)  inch  from  either  end  and  one  and 
three-eighths  (if)  inches  either  way  from  the  longitudinal 
center  line. 


240          A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

'  The  spaces  between  the  staves  will  be  determined 
by  the  accuracy  of  the  heads,  but  must  not  exceed  five- 
sixteenths  (3^-)  inch.  The  interior  or  flat  side  of  each 
channel  must  be  protected  by  a  lining  or  wear  plate 
three-eighths  (f)  inch  thick  by  five  and  one-half  (5^) 
inches  wide  by  nineteen  and  three-fourths  (19!)  inches 
long.  The  wear  plate  shall  consist  of  medium  steel 
plate,  and  shall  be  riveted  to  the  channel  by  three  (3) 
one-half  (J)  inch  rivets,  one  of  which  shall  be  on  the 
center  line  both  ways  and  the  other  two  on  the  longitudinal 
center  line  and  spaced  seven  (7)  inches  from  the  center 
each  way.  The  rivet  holes  shall  be  countersunk  on 
the  face  of  the  wear  plate  and  the  rivets  shall  be  driven 
hot  and  chipped  off  flush  with  the  surface  of  the  wear 
plate.  These  wear  plates  shall  be  inspected  from  time 
to  time,  and  if  found  loose  shall  be  at  once  reriveted, 
but  no  wear  plate  shall  be  replaced  by  a  new  one  except 
as  the  whole  set  is  changed.  No  set  of  wear  plates  shall 
be  used  for  more  than  one  hundred  and  fifty  (150)  tests 
under  any  circumstances.  The  record  must  show  the 
date  when  each  set  of  wear  plates  goes  into  service  and  the 
number  of  tests  made  upon  each  set. 

"  The  staves  when  bolted  to  the  head  shall  form  a 
barrel  twenty  (20)  inches  long,  inside  measurement, 
between  wear  plates.  The  wear  plates  of  the  staves 
must  be  so  placed  as  to  drop  between  the  wear  plates  of 
the  heads.  These  staves  shall  be  bolted  tightly  to  the 
heads  by  four  (4)  three-fourths  (f)  inch  bolts,  and  each 
bolt  shall  be  provided  with  lock  nuts,  and  shall  be  inspected 
at  not  less  frequent  intervals  than  every  fifth  (5th)  test 
and  all  nuts  kept  tight.  A  record  shall  be  made  after 
each  such  inspection,  showing  in  what  condition  the  bolts 
were  found. 


BRICK  PAVEMENTS.  241 

THE  FRAME  AND  DRIVING  MECHANISM. 

"  The  barrel  shall  be  mounted  on  a  cast-iron  frame  of 
sufficient  strength  and  rigidity  to  support  same  without 
undue  vibration.  It  should  rest  on  a  rigid  foundation 
and  be  fastened  to  same  by  bolts  at  not  less  than  four 
(4)  points. 

"  It  should  be  driven  by  gearing  whose  ratio  of  driver 
to  driven  should  not  be  less  than  one  (i)  to  four  (4). 
The  countershaft  upon  which  the  driving  pinion  is 
mounted  should  not  be  less  than  one  and  fifteen-sixteenths 
(ijf)  inches  in  diameter  with  bearings  not  less  than  six 
(6)  inches  in  length  and  belt  driven,  and  the  pulley  should 
not  be  less  than  eighten  (18)  inches  in  diameter  and  six 
and  one-half  (6J)  inches  in  face.  A  belt  of  six  (6)  inch 
double-strength  leather,  properly  adjusted,  so  as  to  avoid 
unnecessary  slipping,  should  be  used. 

"  The  National  Paving  Brick  Manufacturers'  Associa- 
tion will  furnish  without  charge  to  all  proper  applicants 
the  complete  drawings  of  a  machine  which  will  meet  the 
above  specifications  and  requirements. 

THE    ABRASIVE    CHARGE. 

"  (a)  The  abrasive  charge  shall  consist  of  two  sizes 
of  cast-iron  spheres.  The  larger  size  shall  be  three  and 
seventy-five  hundredths  (3.75)  inches  in  diameter  when 
new  and  shall  weigh  when  new  approximately  seven  and 
five-tenths  (7.5)  pounds  (3.40  kilos)  each.  Ten  shall  be 
used. 

"  These  shall  be  weighed  separately  after  each  ten 
(10)  tests,  and  if  the  weight  of  any  large  shot  falls  to 
seven  (7)  pounds  (3.175  kilos)  it  shall  be  discarded  and 
a  new  one  substituted;  provided,  however,  that  all  of 


242  A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

the  large  shot  shall  not  be  discarded  and  substituted  by 
new  ones  at  any  single  time,  and  that  so  far  as  possible 
the  large  shots  shall  compose  a  graduated  series  in  various 
stages  of  wear. 

"  The  smaller  size  spheres  shall  be  when  new  one 
and  eight  hundred  seventy-five  thousandths  (1.875)  inches 
in  diameter  and  shall  weigh  not  to  exceed  ninety-five 
hundredths  (.95)  pound  (0.430  kilo)  each.  Of  these 
spheres  so  many  shall  be  used  as  will  bring  the  collective 
weight  of  the  large  and  small  spheres  most  nearly  to 
three  hundred  (300)  pounds,  provided  that  no  small 
sphere  shall  be  retained  in  use  after  it  has  been  worn 
down  so  that  it  will  pass  a  circular  hole  one  and  seventy- 
five  hundredths  (1.75)  inches  in  diameter,  drilled  in  a 
cast-iron  plate  one-fourth  (J)  inch  in  thickness  or  weigh 
less  than  seventy-five-hundredths  (.75)  pound  (or  .34 
kilo).  Further  the  small  spheres  shall  be  tested  by  passing 
them  over  such  an  iron  plate  drilled  with  such  holes, 
or  shall  be  weighed  after  every  ten  (10)  tests,  and  any 
which  pass  through  or  fall  below  specified  weight,  shall 
be  replaced  by  new  spheres,  and  provided,  further,  that 
all  of  the  small  spheres  shall  not  be  rejected  and  replaced 
by  new  ones  at  any  one  time,  and  that  so  far  as  possible 
the  small  spheres  shall  compose  a  graduated  series  in 
various  stages  of  wear.  At  any  time  that  any  sphere  is 
found  to  be  broken  or  defective  it  shall  at  once  be  replaced. 
"  (b)  The  iron  composing  these  spheres  shall  have  a 
chemical  composition  within  the  following  limits : 

Combined  carbon- — Not  less  than  2.50  per  cent. 

Graphitic  carbon— Not  more  than  0.20  per  cent. 

Silicon- — Not  more  than  i  per  cent. 

Manganese — Not  more  than  0.50  per  cent. 

Phosphorus — Not  more  than  0.25  per  cent. 

Sulphur — Not  more  than  0.08  per  cent. 


BRICK  PAVEMENTS.  243 

"  For  each  new  batch  of  spheres  used  the  chemical 
analysis  must  be  furnished  by  the  maker,  or  be  obtained 
by  the  user,  before  introduction  into  the  charge,  and 
unless  the  analysis  meets  the  above  specifications,  the 
batch  of  spheres  shall  be  rejected. 

THE   BRICK  CHARGE 

"  The  number  of  brick  per  charge  shall  be  ten  (10) 
for  all  bricks  of  the  so-called  *  block  size '  whose  dimen- 
sions fall  between  from  eight  (8)  to  nine  (9)  inches  in 
length,  three  (3)  and  three  and  three-fourths  (3!)  inches 
in  breadth  and  three  and  three-fourths  (3})  and  four  and 
one-fourth  (4-^)  inches  in  thickness.  No  block  should  be 
selected  for  test  that  would  be  rejected  by  any  other  re- 
quirements of  the  specifications. 

The  brick  shall  be  clean  and  dried  for  at  least  three 
(3)  hours  in  a  temperature  of  one  hundred  (100)  degrees 
F.  before  testing. 

THE   TEST 

"  The  rattler  shall  be  rotated  at  a  uniform  rate  of  not 
less  than  twenty-nine  and  one-half  (29^)  nor  more  than 
thirty  and  one-half  (30^)  revolutions  per  minute,  and 
eighteen  hundred  (1800)  revolutions  shall  constitute  the 
standard  test. 

"A  counting  machine  shall  be  attached  to  the  rattler 
for  counting  the  revolutions. 

"A  margin  of  not  to  exceed  ten  (10)  revolutions  will 
be  allowed  for  stopping. 

STOPPING   AND   STARTING 

"Only  one  (i)  start  and  stop  per  test  is  regular  and 
acceptable. 


244         A  TEXT-BOOK  ON  ROADS   AND   PAVEMENTS. 
THE   RESULTS 

"  The  loss  shall  be  calculated  in  percentage  of  the 
original  weight  of  the  dried  brick  composing  the  charge. 
In  weighing  the  rattled  brick  any  piece  weighing  less 
than  one  (i)  pound  shall  be  rejected. 

RECORDS 

"  (a)  The  operator  shall  keep  an  official  book,  in  which 
the  alternate  pages  are  perforated  for  removal.  The 
record  shall  be  kept  in  duplicate,  by  use  of  a  carbon 
paper  between  the  first  and  second  sheets,  and  when 
all  entries  are  made  and  calculations  are  completed  the 
original  record  shall  be  removed  and  the  carbon  duplicate 
preserved  in  the  book.  All  calculations  must  be  made 
in  the  space  left  for  that  purpose  in  the  record  blank, 
and  the  actual  figures  must  appear.  The  record  must 
bear  its  serial  number,  and  be  filled  out  completely  for 
each  test,  and  all  data  as  to  dates  of  inspection  and  weighing 
of  shot  and  replacement  of  worn-out  parts  must  be  care- 
fully entered,  so  that  the  records  remaining  in  the  book 
constitute  a  continuous  one.  In  event  of  further  copies 
of  a  record  being  needed,  they  may  be  furnished  on  separate 
sheets,  but  in  no  case  shall  the  original  carbon  copy  be 
removed  from  the  record  book. 

"  (b)  The  blank  form  upon  which  the  record  of  all 
official  brick  tests  is  to  be  kept  and  reported  is  as  follows: 


BRICK   PAVEMENTS. 


245 


REPORT     OF     STANDARD     RATTLER     TEST     OF     PAVING 
BRICKS. 

IDENTIFICATION  DATA. 

Name  of  the  firm  furnishing  sample Serial  No.  (      ) 

Name  of  the  firm  manufacturing  sample 

Street  or  job  which  sample  represents 

Brands  or  marks  on  the  brick 

Quantity  furnished .  . '. Drying  .treatment 

Date  received Date  tested 

Length Breadth Thickness 

STANDARDIZATION  DATA. 
Number  of  charges  tested  since  last  inspection 


Weight  of  Charge 
(After  Standardization). 

Condition  of  Locknuts 
on  Staves. 

Condition  of  Scales. 

10  Large  spheres 

Small  spheres 

Total      

Number  of  charges  tested  since  stave  linings  were  renewed .... 
Repairs  (Note  any  repairs  affecting  the  condition  of  the  barrel) 

RUNNING  DATA. 


Time  Readings. 

Revolution 
Counter 
Readings. 

Running  Notes. 
Stops,  etc. 

Hours 

Min. 

Sec. 

Beginning  of  test 
Final  reading  .  .  . 

WEIGHTS  AND  CALCULATIONS. 


Initial  weight  of  10  bricks.  . 
Final  weight  of  same  

Percentage  loss 
(Note.  —  The  calculation 
must  appear) 

Los5  of  weight 

Number  of  broken  bricks  and  remarks  on  same  .  . 

I  certify  that  the  foregoing  test  was  made  under  the  specifications 

of and  is  a  true  record. 

(Signature  of  Tester) 


Date Location  of  Laboratory 


246       A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 


ABSORPTION   TEST. 

This  test  is  made  by  weighing  the  specimen  dry, 
then  saturating  it  with  water,  weighing  again,  and 
stating  the  absorption  as  a  percentage  of  the  dry 
weight.  The  Commission  of  the  Brick  Manufacturers' 
Association  oppose  the  use  of  this  test,  but  recommend 
the  following  procedure  for  the  test  when  used: 

I.  The  number  of  bricks  for  a  standard  test  shall 
be  5. 

II.  The  test  must  be  conducted  on  rattled  bricks. 
If  none  such  are  available,  the  whole  bricks  must  be 
broken  in  halves  before  treatment. 

III.  Dry  the  bricks  for  48  hours  at   a  temperature 
ranging  from    230   degrees   to    250    degrees  F.   before 
weighing  for  the  initial  dry  weight. 

IV.  Soak  for  48  hours,   completely  immersing  the 
brick. 

V.  After  soaking   and   before   reweighing  wipe  the 
brick   until   free   from   surplus   water   and   practically 
dry  on  the  surface. 

VI.  Reweigh  the  samples  at  once  on  scales  which 
are  sensitive  to  I  gram. 

VII.  The  increase  in  weight  due  to  absorption  is  to 
be  calculated  in  percentage  of  the  dry  weight  of  the 
original  bricks. 

The  commission  also  adopted  the  following  resolu- 
tion: 

"Resolved,  That,  in  the  opinion  of  the  commission, 
any  paving-brick  which  will  satisfy  the  requirements 
of  reasonable  mechanical  tests  will  not  absorb  sufficient 
water  to  prove  injurious  to  it  in  service.  We  therefore 
recommend  that  the  absorption  test  be  abandoned  as 
unnecessary,  if  not  actually  misleading." 


BRICK  PAVEMENTS.  247 

The  purpose  of  this  test,  when  made,  is  to  insure 
the  proper  burning  of  the  brick  to  a  compact  and  non- 
absorbent  structure.  It  is  probable,  as  claimed  by  the 
commission,  that  these  qualities  will  always  be  shown 
by  the  other  tests,  and  that  this  one  is  not  of  very 
great  importance,  but  in  many  instances  it  may  give 
useful  information.  A  good  paving-brick  will  not 
usually  absorb  more  than  4  per  cent  or  5  per  cent  of 
water,  but  the  amount  of  absorption  depends  largely 
upon  the  nature  of  the  material  from  which  the  brick 
is  made.  Many  of  the  shale  bricks  absorb  less  than 
I  per  cent  of  water  if  properly  burned,  while  some  ot 
the  so-called  fire-clay  bricks  when  of  equally  good 
quality  will  absorb  3  per  cent  or  4  per  cent.  A 
specification  which  would  insure  the  proper  burning 
of  the  one  class  would  exclude  the  best  of  the  other 
class.  A  limit  to  the  amount  of  absorption  allowable 
is,  however,  commonly  set  in  specifications. 

The  requirement  of  drying  48  hours  is  probably,  in 
most  instances,  sufficient  and  reduces  the  moisture  in 
the  brick  so  that  the  further  loss  from  continued 
drying  would  be  very  slight,  but  the  saturation  of  the 
brick  will  not  usually  be  accomplished  by  immersing 
for  48  hours.  Some  bricks  \vill  in  that  time  have 
taken  up  but  a  small  part  of  the  water  they  would 
finally  absorb,  and  much  longer  time  would  be  neces- 
sary to  give  a  complete  indication  in  this  particular. 
Some  experiments  by  Mr.  Harrington  of  St.  Louis, 
the  results  of  wThich  were  presented  to  the  Brick 
Manufacturers'  Association,  showed  a  considerable 
change  in  the  quantity  of  water  absorbed  by  some 
bricks  through  a  period  of  24  weeks,  and  a  consider- 
able variation  in  the  rate  of  absorption  by  different 
bricks. 


248      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

The  application  of  the  test  as  proposed  may  serve 
to  show  whether  the  absorption  is  within  the  proper 
limits  for  a  paving-brick,  and  when  properly  applied 
to  particular  makes  of  brick  may  indicate  the  degree 
of  burning,  although  it  is  not  of  much  value  in  the 
comparison  of  the  qualities  of  different  bricks  where 
each  shows  results  within  reasonable  limits. 


SPECIFIC-GRAVITY  TEST. 

A  test  for  specific  gravity  is  sometimes  included 
in  specifications  with  a  view  to  insuring  the  burning  of 
•the  brick  to  the  proper  density. 

In  consequence,  however,  of  the  variation  of  bricks 
made  from  different  materials  it  does  not  seem  feasible 
to  adopt  any  requirement  foi  general  use  which  would 
be  of  much  value,  and  as  the  same  qualities  are  de- 
termined by  other  tests  it  seems  unnecessary.  The 
Commission  of  the  Brick  Manufacturers'  Association 
recommend  the  abandonment  of  this  test. 

COMPARISON    OF   TESTS. 

Specifications  for  brick  pavements  commonly  require 
that  the  brick  reach  certain  limiting  values  on  some  of 
the  tests  which  have  been  described.  The  crushing  test 
and  the  specific  gravity  test,  while  useful  sometimes  in  a 
study  of  the  character  of  materials,  are  of  no  value  in 
specifications  and  are  seldom  used.  Absorption  is  some- 
times specified,  a  limit  of  2  to  4  per  cent  being  given  for 
the  shale  brick,  but  is  a  doubtful  value  as  a  measure  of 
quality.  The  transverse  test  is  frequently  used,  requiring 
about  1800  to  2000  pounds  per  square  inch  as  modulus 
of  rupture  by  the  standard  test. 


BRICK  PAVEMENTS  249 

The  main  dependence,  in  specifications,  is  usually 
placed  upon  the  rattler  test,  which  is  without  doubt  the 
most  important,  as  showing  more  nearly  than  the  others 
the  qualities  necessary  in  a  brick  to  give  good  wear 
under  traffic  in  a  pavement,  and  many  specifications  now 
require  no  other  test  for  acceptance  of  the  brick.  Speci- 
fications vary  in  the  percentage  of  loss  allowed,  in  testing 
by  the  old  method,  from  about  1 8  to  27  per  cent,  according 
to  brick  available  in  the  locality,  the  intensity  of  traffic, 
and  the  experience  of  the  engineer  with  the  test.  In 
adopting  the  new  test  the  American  Society  of  Municipal 
Improvements  recommend  that  the  limit  of  loss  of  weight 
be  fixed  at  22  per  cent  for  heavy,  26  per  cent  for  medium, 
and  28  per  cent  for  light  traffic  streets.  The  Association 
for  Standardizing  Paving  Specifications  recommends  that 
the  brick  shall  not  lose  more  than  22  per  cent  of  their 
weight,  but  provides  that  "  where  medium  or  light  traffic 
or  other  conditions  which  in  the  opinion  of  the  engineer 
do  not  require  a  brick  sufficient  to  stand  an  abrasion 
loss  of  22  per  cent,  brick  of  a  quality  sufficient  to  stand 
a  loss  of  25  per  cent  or  even  28  per  cent  may  be  used." 

ART.  63.    CONSTRUCTION  OF  BRICK   PAVEMENTS. 

The  work  to  be  performed  in  laying  a  brick  pave- 
ment, after  grading  and  rolling  the  road  bed  (see 
Art.  53),  consists  in  placing  the  foundation;  forming  a 
cushion  coat  of  sand  over  the  foundation;  laying  the 
bricks  upon  the  sand  cushion;  rolling,  or  ramming,  the 
bricks  to  a  uniform  surface  and  bearing;  culling  all 
broken  and  imperfect  bricks ;  filling  the  joints  between 
the  bricks;  cleaning  the  pavement  and  opening  it  to 
traffic. 

Foundation.     A  brick  pavement  should  have  a  firm 


250      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

foundation.  As  the  surface  is  made  up  of  small  inde- 
pendent blocks,  each  brick  must  be  adequately  sup- 
ported from  below,  or  the  loads  coming  upon  it  may 
force  it  downward  and  cause  unevenness.  The  wear  of 
the  pavement  depends  very  largely  upon  the  mainten- 
ance of  a  smooth  even  surface,  as  any  unevenness  will 
cause  the.  bricks  to  chip  on  the  edges,  and  also  produce 
impact  from  the  loads  passing  over  the  pavement . 

The  best  foundation  for  a  brick  pavement  is  doubt- 
less one  of  concrete,  laid  after  the  manner  given  in 
Art.  57.  For  light  or  moderately  heavy  traffic,  such  as 
that  of  the  ordinary  small  city  where  the  road-bed  is 
of  firm  soil  and  properly  drained,  the  concrete  is  usu- 
ally placed  4  to  6  inches  thick.  If  the  traffic  be  very 
heavy  or  where  from  any  cause  the  road-bed  is  not  firm, 
it  may  be  advisable  to  still  farther  increase  the  depth. 

The  double-layer  pavement  (see  Fig.  24)  consists  of 
a  foundation  made  by  placing  a  layer  of  sand  or  gravel 
3  to  5  inches  thick  upon  the  road-bed,  rolling  it 
thoroughly  and  laying  a  course  of  bricks  upon  it.  The 
bricks  are  laid  flat  with  their  greatest  dimension  length- 
wise of  the  street,  as  explained  in  Art.  59.  This  founda- 
tion has  been  extensively  used  under  brick  pavements, 
and  has  often  given  satisfactory  results.  It  is  now  largely 
giving  place  to  concrete  in  the  better  class  of  work,  and 
in  many  cases  under  light  traffic  its  economy  is  question- 
able, as  the  layer  of  gravel  would  often  answer  equally 
well  without  the  lower  layer  of  bricks.  A  modification  of 
this  base  has  been  used  in  a  few  instances,  in  which  the 
joints  between  the  bricks  are  filled  with  Portland  cement 
mortar.  When  such  a  base  can  be  placed  cheaper  than 
concrete,  it  may  give  good  service.  The  National  Paving 
Brick  Manufacturers'  Association  propose  the  following 
method  of  construction : 


BRICK  PAVEMENTS.  251 

"  Upon  the  sub-grade  as  heretofore  prepared  shall  be 
spread  a  base  of  sand  two  (2)  inches  in  thickness  and 
which  shall  be  brought  to  a  perfect  grade,  conforming 
to  that  of  the  finished  street. 

"  There  shall  be  laid  flatwise  at  right  angles  with  the 
street,  upon  this  grade  thus  prepared,  a  layer  of  No.  2 
Paving  Block  not  less  than  three  (3)  inches  in  thickness, 
the  interstices  of  which  shall  be  rilled  with  a  filler  com- 
posed of  two  parts  of  clean  sand  and  one  part  of  Portland 
cement.  This  filler  shall  be  prepared  and  applied  as 
provided  for  in  Section  Ten  of  this  direction  and  specifi- 
cation. The  foundation  thus  made  should  remain  undis- 
turbed at  least  thirty-six  (36)  hours  before  the  sand 
cushion  herein  provided  for  may  be  spread,  and  at  least 
ten  (10)  days  must  elapse  before  rolling  and  compacting 
of  brick  surface  is  allowed,  and  in  no  event  must  teams 
be  permitted  or  hauling  be  allowed  upon  this  surface 
during  this  period." 

Sand  Cushion.  The  sand  cushion  consists  usually 
of  a  layer  of  sand  varying,  in  the  practice  of  different 
engineers,  from  i  to  2\  inches  in  thickness  over  the 
surface  of  the  foundation.  The  most  common  prac- 
tice is  to  make  the  sand  cushion  2  inches  thick.  It 
should  be  deep  enough  to  admit  of  the  brick  being 
driven  to  a  smooth  surface,  and  to  take  up  any  in- 
equalities in  the  surface  of  the  foundation  and  differ- 
ences in  thickness  of  brick.  When  a  very  thin  cushion 
layer  is  employed  it  is  necessary  to  secure  much 
greater  accurac}^  in  forming  the  surface  of  the  founda- 
tion, and  it  is  much  more  difficult  to  uniformly  bed 
the  bricks  than  when  the  usual  thickness  is  used.  It 
has  also  been  claimed  that  the  thicker  sand-bed  has 
a  marked  tendency  to  diminish  the  rumbling  of  the 
pavement.  This,  however,  is  perhaps  rather  doubtful 


t 

252         A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

In  forming  the  sand-bed  the  sand  is  spread  over  the 
foundation  a  little  deeper  than  the  bed  is  to  be  left,  and 
is  then  drawn  off  to  a  smooth  surface  by  the  use 
of  a  form,  cut  to  the  desired  shape  of  the  surface, 
which  extends  across  the  street,  and  slides  on  the 
curbs  or  on  stringers  laid  lengthwise  of  the  street  as 
may  be  convenient.  The  making  of  a  good  firm  bed 
requires  that  considerable  sand  shall  be  pulled  off 
from  all  parts  of  the  bed,  and  the  sand  should  always 
be  two  or  three  inches  deep  against  the  front  of  the 
form  when  drawing  it  to  cut  the  bed.  In  order  to 
accomplish  this,  without  failing  to  cut  a  perfect  sur- 
face through  the  form  leaving  the  guides  and  riding 
up  on  the  sand,  considerable  weight  is  required  on  the 
form  when  drawing  it.  In  some  cases  the  sand  cushion 
is  rolled  with  a  light  roller  after  being  formed  with  a 
template.  The  specifications  of  the  Association  for 
Standardizing  Paving  Specifications  require  that  this  be 
done. 

The  sand  for  a  cushion  should  be  clean  and  free 
from  pebbles,  which  prevent  the  formation  of  a  smooth 
bed,  and  possibly  also  cause  the  breaking  of  the  bricks 
in  ramming  the  surface. 

When  the  sand  cushion  is  ij  or  2  inches  deep,  an 
allowance  of  about  half  an  inch  is  necessary  for  settle- 
ment in  driving  the  bricks  to  surface. 

Laying  the  Brick.  The  bricks  in  a  street  pavement 
are  usually  laid  on  edge  in  courses  across  the  street, 
each  alternate  course  being  begun  with  a  half-brick  to 
break  joints  in  the  courses.  This  is  illustrated  in 
Fig.  23,  which  represents  a  pavement  as  constructed  for 
heavy  traffic  on  concrete  foundation. 

In  many  cases  the  gutter-bricks  are  turned  with  the 
greatest  dimension  lengthwise  of  the  street,  with  the 


BRICK  PAVEMENTS. 


253 


object    of    facilitating     the    flow     of    surface-water    in 
the    gutter.      The   advantage   of    this  is  doubtful,  as  it 


FIG.  23. 

has  the  effect  of  breaking  the  bond  of  the  pavement 
between  the  gutter-bricks  and  roadwa}7.  This  is  shown 
in  Fig.  24,  which  represents  the  construction  of  a 


FIG.  24. 

double-layer  pavement  with  brick  and  gravel  base,  as 
has  been  commonly  used  under  light  or  moderate 
traffic. 

In  laying  the  bricks  the  men  stand  on  the  pavement 
already  laid  and,  beginning  at  a  curb,  lay  three  or  four 
courses  across  the  street  at  once,  the  bricks  being 
wheeled  and  piled  on  the  edge  of  the  finished  work 


254      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

by  laborers  working  continually  in  advance  of  the  men 
laying.  Wheeling  over  the  newly  laid  bricks  should 
be  done  on  planks,  to  prevent  driving  the  bricks  out 
of  surface.  The  courses  may  be  kept  straight  and 
close  together  by  driving  back  each  block  of  eight  or 
ten  courses,  a  straight  piece  of  plank  four  or  five  feet 
long  being  held  by  a  handle  on  top  against  the  side  of 
the  last  course  of  bricks,  and  tapped  lightly  with  a  sledge. 

Surfacing  the  Pavement.  After  the  surface  layer  of 
bricks  is  in  position  it  should  be  swept  clean  and  rammed 
or  rolled  to  a  smooth  and  uniform  surface.  A  five- 
or  six-ton  roller  may  be  employed,  passing  three  or 
four  times  over  the  surface,  or  a  wooden  rammer  loaded 
with  lead  to  a  weight  of  80  or  100  pounds  may  be  used 
by  striking  upon  a  plank  laid  lengthwise  of  the  street. 
The  plank  should  be  10  or  12  feet  long  by  about  a  foot 
wide  and  3  inches  thick,  and  used  only  so  long  as  it 
retains  its  form  and  solidity. 

The  National  Paving  Brick  Manufacturers'  Association 
recommends  the  following  procedure  in  compacting  and 
smoothing  the  surface  of  the  pavement: 

"  After  the  brick  in  the  pavement  are  inspected  and 
the  surface  is  swept  clean  of  spalls,  they  must  be  well 
rolled,  with  a  steam  roller  weighing  not  less  than  three 
nor  more  than  five  tons,  in  the  following  manner:  The 
brick  next  the  curb  should  be  tamped  with  a  hand  wood 
tamper  to  the  proper  gutter  grade.  The  rolling  will 
then  commence  near  the  curb  at  a  very  slow  pace  and 
continue  back  and  forth  until  the  center  of  the  pavement 
is  reached,  then  pass  to  the  opposite  curb  and  repeat 
in  the  same  manner  to  the  center  of  the  street.  After 
this  first  passage  of  the  roller  the  pace  may  be  quickened 
and  the  rolling  continued  until  each  brick  is  firmly 
imbedded  in  the  sand  cushion.  The  roller  shall  then  be 


BRICK  PAVEMENTS.  255 

started  at  the  end  of  the  block  and  the  pavement  rolled 
transversely  at  an  angle  of  45  degrees  to  curb,  repeat  the 
rolling  in  like  manner  in  the  opposite  direction.  Before 
this  transverse  rolling  takes  place  all  broken  or  injured 
brick  must  be  taken  up  and  replaced  with  perfect  ones." 

When  the  pavement  has  been  brought  to  surface  a 
careful  inspection  should  be  made  and  all  defective 
or  broken  brick  removed  and  replaced,  a  pair  of  brick- 
tongs  being  used  for  the  purpose,  and  all  low  bricks 
or  low  spots  being  raised  and  brought  to  surface.  A 
straight-edge  is  desirable  in  determining  surface,  as 
the  appearance  is  often  deceptive  to  the  eye,  and  a 
slight  variation  in  color  of  brick  is  frequently  mistaken 
for  an  irregularity  of  surface.  Sometimes  the  pavement 
is  sprinkled  and  soft  bricks  picked  out  by  observing 
whether  they  hold  moisture;  this  method  should  be 
used  with  caution  and  with  full  knowledge  of  the 
material,  as  sometimes  a  comparatively  slight  absorp- 
tion will  show  quite  markedly. 

ART.  64.    FILLING  THE  JOINTS. 

There  is  much  difference  of  opinion  amongst  munici- 
pal engineers  concerning  the  best  material  to  use  in 
filling  the  joints  in  a  brick  pavement.  The  materials 
commonly  employed  are  sand,  Portland-cement  grout, 
and  asphaltic  or  coal-tar  paving-cement.  Certain  patent 
fillers  of  more  or  less  the  same  character  are  also  some- 
times employed. 

Sand  Filler.  In  using  sand  as  a  filler,  a  thin  layer 
of  sand  is  spread  over  the  pavement  and  raked  or 
swept  into  the  joints  until  they  are  thoroughly  filled. 
In  some  instances  the  sand  is  artificially  dried  before 
putting  it  upon  the  bricks,  but  ordinarily,  in  mod- 


256       A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

erately  dry  weather,  the  sand  may  be  spread  in  a  thin 
layer  on  the  bricks  and  allowed  to  dry  before  sweep- 
ing in.  After  the  joints  are  well  filled  a  light  layer 
of  sand  is  placed  on  top  of  the  pavement  and  it  is 
opened  to  traffic.  The  jarring  of  the  traffic  will 
cause  the  sand  to  settle  more  or  less  in  the  joints  for 
a  considerable  time,  and  the  sand  cover  should  be 
retained  for  several  weeks.  The  sand  used  for  filling 
should  be  fine  and  sharp,  free  from  loam  and  dirt.  It 
must  not  pack  or  cake  on  top  of  the  brick  under  traffic. 
In  many  instances  sand  has  been  used  as  a  filler  with 
satisfactory  results  and  given  good  service  even  under 
moderately  heavy  traffic.  It  makes  a  practically 
impervious  joint  and  holds  the  bricks  quite  firmly  in 
place.  It  seems  desirable  in  the  use  of  sand  filler  to 
employ  round-edge  bricks,  as  the  edges  are  not  held  so 
firmly  as  with  a  rigid  filler,  and  if  sharp  are  more  likely 
to  be  chipped  off,  while  the  round  edges  aid  in  thoroughly 
filling  the  joints  with  the  sand. 

Portland-cement  Filler.  For  filling  joints  with  Port- 
land cement  a  grout  composed  of  equal  parts  of  cement 
and  sand  is  commonly  employed.  This  grout  is  mixed 
in  a  tight  box  to  a  condition  such  that  it  will  readily 
flow  into  the  joints,  and  is  swept  in  with  brooms  until 
the  joints  are  thoroughly  filled.  The  easiest  method 
of  securing  the  complete  filling  of  the  joint  is  probably 
that  of  applying  the  grout  in  two  parts,  mixing  the  first 
part  very  wet  and  filling  the  joints  nearly  full.  As  the 
grout  begins  to  stiffen,  the  draining  out  of  the  water 
causes  it  to  settle  somewhat,  but  thoroughly  fills  and 
seals  the  lower  part  of  the  joint.  The  second  part  may 
then  be  mixed  a  little  stiffer  and  the  upper  part  of 
the  joint  be  readily  filled  flush  with  the  top  of  the 
bricks. 


BRICK  PAVEMENTS.  257 

In  handling  the  grout  it  is  necessary  that  it  be  mixed 
quickly  and  applied  at  once  without  giving  it  time  to 
settle,  in  order  that  it  may  retain  its  consistency  and 
no  separation  of  its  materials  take  place.  A  pavement 
so  filled  becomes  practically  a  monolithic  mass,  as  the 
bricks  are  firmly  held  together  and  the  joint  is  filled 
flush  with  the  edges  of  the  bricks  with  a  material  which 
soon  becomes  about  as  hard  as  the  bricks  themselves. 
The  National  Paving  Brick  Manufacturers'  Association 
advocates  the  exclusive  use  of  this  filler,  and  recommends 
the  following  method  of  applying  it: 

"  The  filler  shall  be  composed  of  one  part  each  of  clean, 
sharp  sand  and  Portland  cement.  The  sand  should  be 
dry.  The  mixture,  not  exceeding  one-third  bushel  of 
the  sand,  together  with  a  like  amount  of  cement,  shall 
be  placed  in  the  box  and  mixed  dry,  until  the  mass 
assumes  an  even  and  unbroken  shade.  Then  water 
shall  be  added,  forming  a  liquid  mixture  of  the  consistency 
of  thin  cream. 

"  The  side  and  edges  of  the  brick  should  be  thoroughly 
wet  before  the  filler  is  applied  by  being  gently  sprinkled. 

"  From  the  time  the  water  is  applied  until  the  last 
drop  is  removed  and  floated  into  the  joints  of  the  brick 
pavement,  the  mixture  must  be  kept  in  constant  motion. 

"  The  mixture  shall  be  removed  from  the  box  to  the 
street  surface  with  a  scoop  shovel,  all  the  while  being 
stirred  in  the  box  as  the  same  is  being  thus  emptied. 
The  box  for  this  purpose  shall  be  4  feet  8  inches  long, 
30  inches  wide  and  14  inches  deep,  resting  on  legs  of 
different  lengths,  so  that  the  mixtures  will  readily  flow 
to  the  lower  corner  of  the  box,  the  bottom  of  which  should 
be  6  inches  above  the  pavement.  This  mixture,  from 
the  moment  it  touches  the  brick  shall  be  thoroughly 
swept  into  the  joints. 


258      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

"  Two  such  boxes  shall  be  provided  in  case  the  street 
is  twenty  feet  or  less  in  width;  exceeding  twenty  feet  in 
width,  three  boxes  should  be  used.  (See  specifications 
for  making  same.) 

"  The  work  of  filling  should  thus  be  carried  forward 
in  line  until  an  advance  of  fifteen  to  twenty  yards  has 
been  made,  when  the  same  force  and  appliances  shall 
be  turned  back  and  cover  the  same  space  in  like  manner, 
except  to  make  the  proportions  two-thirds  Portland  cement 
and  one -third  sand. 

"  To  avoid  the  possibility  of  thickening  at  any  point 
there  should  be  a  man  with  a  sprinkling  can,  the  head 
perforated  with  small  holes,  sprinkling  gently  the  surface 
ahead  of  the  sweepers. 

"  Within  one-half  to  three-quarters  of  an  hour  after 
this  last  coat  is  applied  and  the  grout  between  the  joints 
has  fully  subsided  and  the  initial  set  is  taking  place, 
the  whole  surface  must  be  slightly  sprinkled  and  all 
surplus  mixture  left  on  the  tops  of  the  brick  swept  Into 
the  joints,  bringing  them  up  flush  and  full. 

"  After  the  joints  are  thus  filled  flush  with  the  top  of 
the  brick  and  sufficient  time  for  hardening  has  elapsed, 
so  that  the  coating  of  sand  will  not  absorb  any  moisture 
from  the  cement  mixture,  one-half  inch  of  sand  shall 
be  spread  over  the  whole  surface,  and  in  case  the  work 
is  subjected  to  a  hot  summer  sun,  an  occasional  sprinkling, 
sufficient  to  dampen  the  sand,  should  be  followed  for 
two  or  three  days." 

When  using  Portland  cement  filler,  an  expansion  joint 
must  be  placed  next  the  curb  on  each  side  of  the  street, 
to  prevent  the  brick  surface  being  lifted  from  the  sand 
cushion  by  expansion.  This  joint  is  made  by  placing 
strips  of  wood  along  the  curb,  against  which  the  brick 
may  be  set,  and  after  the  Portland  cement  filler  has  been 


BRICK  PAVEMENTS.  259 

put  in,  the  boards  are  removed  and  the  expansion  joints 
filled  with  asphaltic  or  coal  tar  cement. 

Sometimes  expansion  joints  are  also  used  at  intervals 
across  the  street  to  provide  for  longitudinal  expansion. 
To  be  efficient,  these  joints  should  not  be  less  than  J 
inch,  when  spaced  50  feet  apart.  They  will  usually  be 
nearly  closed  in  a  few  months,  the  paving  pitch  being 
forced  out  by  the  expansion.  Such  joints  are  not  usually 
necessary  on  pavements  of  uniform  gradients,  but  where 
changes  occur  in  the  rate  of  grade,  and  at  intersections 
expansion  must  be  carefully  provided  for.  After  com- 
pletion, the  pavement  should  stand  at  least  a  week  or 
ten  days  before  it  is  opened  to  traffic,  to  allow  sufficient 
time  for  the  cement  to  harden.  During  this  time  it 
should  not  be  exposed  to  hot  sun  or  permitted  to  dry 
out  too  much;  commonly  a  light  coating  of  sand  is  spread 
over  it,  and  sometimes  it  is  dampened  by  frequent 
sprinklings. 

Bituminous  Filler.  The  bituminous  cements  employed 
as  fillers  for  brick  pavements  may  be  prepared  from  coal 
tar  or  petroleum  residuums,  or  from  natural  asphalt, 
and  are  similar  in  character  to  the  materials  used  for 
bituminous  macadam  or  in  the  preparation  of  asphalt 
surface  mixtures. 

On  account  of  the  variation  in  the  materials  used, 
the  results  obtained  with  bituminous  fillers  have  differed 
widely  in  different  places.  In  many  cases  very  satis- 
factory results  have  been  obtained  with  these  fillers; 
in  others,  the  susceptibility  to  temperature  changes  has 
made  them  failures,  the  material  melting  and  running 
out  of  the  joints  in  hot  weather,  or  becoming  brittle  and 
chipping  out  in  cold  weather. 

Bituminous  fillers,  when  of  good  materials  and  properly 
applied,  give  very  satisfactory  results.  They  have  the 


,  260      A  TEXT-BOOK  ON  ROADS   AND   PAVEMENTS. 

advantage  over  sand  of  being  quite  impervious  to  water, 
while  they  are  not  so  rigid  as  Portland  cement  filler 
and  are  less  noisy.  These  materials  are  under  the  dis- 
advantage that  well-defined  specifications,  which  will 
insure  the  quality  of  the  material,  have  not  been  worked 
out.  Most  of  the  tar  fillers  which  have  been  on  the 
market  are  too  susceptible  to  changes  of  temperature. 
Some  of  the  asphaltic  fillers  have  given  good  results. 
For  asphaltic  fillers,  the  Association  for  Standardizing 
Paving  Specifications  recommends  the  following  require- 
ments : 

'  The  interstices  in  the  brick  shall  be  completely  filled 
with  an  asphalt  filler  heated  to  a  temperature  of  not  less 
than  350°  F.  nor  more  than  450°  F.  This  asphalt  filler 
shall  not  contain  pitch  nor  any  part  of  coal  tar.  It 
shall  contain  at  least  92  per  cent  of  bitumen  soluble  in 
carbon  disulphide.  It  shall  remain  pliable  at  all  tem- 
peratures to  which  it  may  be  subjected  as  a  street 
paving  filler;  it  shall  be  absolutely  proof  against  water 
and  street  liquids;  it  shall  firmly  adhere  to  the  brick 
and  be  pliable  rather  than  rigid.  Care  shall  be  exercised 
to  completely  fill  all  openings  around  street  structures 
and  the  street  shall  not  be  used  for  traffic  until  the 
filler  is  completely  set.  A  top  dressing  of  sand  shall  be 
spread  immediately  after  the  filler  is  applied  and  while 
it  is  still  soft." 

"  The  penetration  shall  conform  to  the  following: 

"No.  2  needle  5  sec.    100  grams  at     77  degrees  F.,  25  to  60. 
"No.  2  needle  i  min.  200  grams  at     32  degrees  F.,  not  below  25. 
"No.  2  needle  5  sec.     50  grams  at   115  degrees  F.,  not  below  no." 

When  bituminous  filler  is  employed  it  is  melted  in 
kettles  on  the  street  and  poured  hot  into  the  joints. 
The  paving-cement  is  applied  at  a  temperature  of  300° 


BRICK  PAVEMENTS.  261 

to  400°  F.,  and  should  be  applied  only  when  the  bricks 
are  dry.  After  the  joints  are  filled  a  light  layer  of  sand 
is  spread  over  the  surface,  and  serves  under  the  traffic 
to  clean  the  surface  from  any  surplus  bitumen  which 
may  be  smeared  over  it. 

ART.  65.    MAINTENANCE  OF  BRICK  PAVEMENTS. 

The  maintenance  necessary  for  a  brick  pavement 
consists  in  keeping  it  clean  and  carefully  watching  it, 
especially  during  the  first  year  or  two  years,  to  see  that 
no  breaks  occur  due  to  the  use  of  defective  bricks  in 
the  surface  or  to  insufficient  support  from  the  founda- 
tion at  any  point.  When  any  unevenness  from  either 
of  these  causes  appears,  it  should  be  at  once  rectified 
before  the  pavement  becomes  irregularly  worn  in  con- 
sequence. 

While,  as  already  stated,  the  utmost  care  should 
always  be  taken  to  use  only  material  of  a  uniform 
quality  in  the  surface  of  the  pavement,  still  under  the 
closest  inspection  some  inferior  material  may  be  used, 
which  will  only  be  shown  when  wear  comes  on  the 
pavement,  and  unless  then  removed  at  once  it  will 
cause  the  evenness  of  the  surface  to  be  impaired  about 
it.  Irregular  support  from  the  foundation  will  be 
less  likely  to  occur  in  good  construction,  but  its  effect 
will  be  similar  to  defective  material,  the  sinking  of  in- 
dividual bricks  producing  uneven  wear.  Weak  spots 
in  the  foundation  may  sometimes  be  caused,  where  con- 
crete foundation  is  not  employed,  by  surface-water 
which  is  permitted  to  pass  through  the  joints,  saturat- 
ing the  sand  or  gravel  beneath  and  causing  it  to  move 
under  concentrated  loads.  For  this  reason  the  joints 
should  be  observed  during  the  early  wear  of  the  pave- 


262       A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

ment  in  order  to  remedy  any  case  where  they  may  not 
have  been  properly  filled. 

Where  a  brick  pavement  has  been  constructed  of 
good  material  and  kept  in  good  surface  during  the 
early  period  of  use,  it  may  then  reasonably  be  expected 
to  wear  out  without  any  considerable  expense  for  small 
repairs.  The  length  of  time  the  pavement  may  be 
expected  to  wear  depends  upon  the  quality  of  the 
materials  and  the  method  of  construction.  For  the 
heavier  traffic  of  many  of  the  smaller  cities,  and 
streets  of  moderate  traffic  in  the  larger  cities,  brick  has 
shown  an  endurance  which  indicates  it  to  be  a  satis- 
factory and  economical  material. 

In  contracting  for  the  construction  of  brick  pave- 
ments, many  cities  require  the  contractor  to  guarantee 
the  pavement  for  a  term  of  years,  making  all  necessary 
repairs  during  the  period  of  guaranty.  This  is  intended 
as  an  assurance  of  the  quality  of  the  work.  A  guaranty 
of  the  pavement  for  one  year  may  often  be  of  use  in 
discovering  any  serious  defects  in  construction,  and 
will  not  add  materially  to  the  cost,  but  the  engineer 
in  charge  of  the  work  has  means  of  accurately  judging 
its  quality  and,  where  a  long  period  of  maintenance  is 
required,  it  is  doubtful  whether  the  gain  in  quality  is 
sufficient  to  warrant  the  increase  in  price  necessitated 
by  the  guaranty. 


CHAPTER  IX. 

ASPHALT  PAVEMENTS. 

ART.  66.     ASPHALT 

THE  term  asphalt,  as  commonly  used,  includes  all  of 
the  solid  bitumens  which  are  used  in  the  construction  of 
street  pavements.  Some  account  of  the  classification  and 
characters  of  these  bitumens  has  been  given  in  Chapter 
VI.  The  materials  usually  classed  as  asphalts  include 
the  true  asphalts,  gilsonite,  and  grahamite.  Some  residual 
pitches  derived  from  the  distillation  of  petroleums  are 
also  frequently  called  asphalts.  In  California,  the 
residual  pitches  derived  from  asphaltic  petroleums,  as 
prepared  for  use  in  paving,  are  known  as  "  D  "  grade 
aspJialts,  while  in  the  East,  residual  pitches  from  some 
of  the  semi-asphaltic  petroleums,  used  for  road  binders, 
are  sometimes  called  oil  asphalts. 

The  natural  asphalts  used  in  paving  in  the  United  States 
are  obtained  mainly  from  Trinidad  and  Venezuela, 
although  they  are  found  to  some  extent  in  the  United 
States,  Cuba,  and  Mexico.  The  most  important  supplies 
of  native  solid  bitumen  in  the  United  States  are  the 
gilsonites,  which  are  being  used  to  considerable  extent 
for  street  pavements.  Grahamite  has  also  been  used 
in  a  small  way. 

TRINIDAD    ASPHALT. 

The  most  important  source  of  supply  of  asphalt  for 
street  pavements  in  the  United  States  is  that  of  the 

263 


264       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

island  of  Trinidad,  W.  I.  This  asphalt  is  known  as 
lake  asphalt  or  land  asphalt,  according  to  the  source 
from  which  it  is  obtained.  Lake  asphalt  is  found  in  a 
large  deposit  known  as  the  pitch  lake.  This  lake  covers 
an  area  of  over  100  acres,  and  lies  in  a  deep  crater  with 
steeply  sloping  sides.  The  pitch  seems  to  be,  or  to  have 
been,  forced  up  from  below,  and  it  is  more  or  less  in 
motion,  excavations  in  the  surface  being  gradually  filled 
by  flow  of  material  from  sides  and  bottom.  Upon 
exposure  to  the  air,  the  pitch  slowly  hardens,  is  some- 
what softer  near  the  center  of  the  lake  than  at  the  sides, 
and  it  has  been  supposed  that  the  supply  from  sub- 
terranean sources  still  continues  to  some  extent.  It  has 
also  been  found  that  the  surface  of  the  lake  is  higher 
in  the  center  than  at  the  sides,  and  that  the  general 
elevation  of  the  surface  has  been  lowered  somewhat  by 
the  large  quantities  of  material  which  have  been  removed 
from  it.* 

In  Trinidad  lake  asphalt,  the  bitumen  occurs  mixed 
with  considerable  quantities  of  finely  divided  mineral 
matter,  as  well  as  with  water  and  small  amounts  of  other 
impurities.  In  order  to  remove  this  water  and  any 
vegetable  impurities  which  the  crude  material  may  con- 
tain, the  asphalt  is  refined  by  heating  sufficiently  to 
vaporize  the  water  and  melt  the  bitumen.  This  is  accom- 
plished either  by  the  use  of  a  large  kettle  heated  directly 
by  fire,  or  by  passing  steam  through  pipes  inside  the  tank 
containing  the  asphalt.  During  the  heating  the  material 
is  agitated  by  a  current  of  air  or  steam.  When  the  water 
has  been  driven  off,  and  the  material  is  thoroughly  melted, 
the  liquid  asphalt  is  drawn  off  and  is  known  as  refined 

*  For  a  complete  description  of  the  Trinidad  pitch  deposits  see  the 
"Report  of  the  Inspector  of  Asphalts  and  Cements  of  the  District  of 
Columbia,"  for  1891-92. 


ASPHALT  PAVEMENTS.  265 

asphalt.  In  refining  the  asphalt  no  effort  is  made  to 
remove  the  mineral  matter,  which  is  present  in  a  finely 
divided  state,  and  is  utilized  to  replace  sand  or  dust 
which  must  otherwise  be  added  in  forming  the  paving 
mixture. 

Refined  Trinidad  Lake  AspJwli  consists  ordinarily  of 
about  54  per  cent  to  57  per  cent  bitumen,  5  per  cent  to 
8  per  cent  of  organic  matter  not  soluble  in  carbon  bisul- 
phide, and  35  per  cent  to  38  per  cent  of  mineral  matter. 
The  bitumens  contain  about  63  per  cent  to  66  per  cent 
of  malthenes  (according  to  Richardson's  classification), 
the  remainder  being  asphaltenes,  with  sometimes  about 
i  per  cent  of  carbenes.  These  asphaltenes  contain  con- 
siderable sulphur  and  are  hard,  brittle  substances,  which 
do  not  melt,  but  are  readily  soluble  in  the  asphaltic  oils. 
The  non-bituminous  organic  matter  is  mainly  material 
which  seems  to  have  been  formed  through  oxidation  of 
some  of  the  harder  bitumens  of  the  asphalt.  It  contains 
a  considerable  amount  of  sulphur  and  may  be  considered, 
like  the  finely  divided  mineral  matter,  as  of  use  as  filler. 
The  mineral  matter  in  Trinidad  asphalt  is  found  in  a 
finely  pulverized  condition  and  quite  uniformly  distributed 
through  the  mass. 

Trinidad  Land  AspJtalt  is  found  in  numerous  deposits 
in  vicinity  of  the  pitch  lake,  most  of  them  covered  with 
soil.  These  deposits  may  have  been  formed  either  from 
the  overflow  of  the  lake  or  from  independent  sources,  the 
action  of  which  has  long  since  ceased.  The  character 
of  the  land  asphalt  is  more  variable  than  that  of  the 
lake,  and  seems  to  depend  upon  the  length  of  time  it 
has  been  exposed  to  the  weather.  The  bitumen  of  the 
asphalt  undergoes  a  gradual  hardening  with  time,  the 
percentage  of  malthenes  becoming  less  as  compared  with 
that  of  the  asphaltenes.  In  some  instances  the  amount 


266       A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

of  mineral  matter  is  greater,  while  the  non-bituminous 
organic  matter  is  increased  by  the  changing  of  some 
of  the  bitumen  to  an  insoluble  condition. 

Refined  Trinidad  Land  Asphalt  consists  commonly  of 
about  51  per  cent  to  55  per  cent  bitumen,  7  per  cent 
to  10  per  cent  organic  matter  insoluble  in  carbon  bisul- 
phide, and  37  per  cent  to  40  per  cent  of  mineral  matter. 
The  bitumen  contains  from  50  per  cent  to  63  per  cent 
of  malthenes,  soluble  in  88°  naphtha  solution.  In  the 
use  of  the  land  asphalts,  larger  quantities  of  fluxing 
materials  must  be  employed  to  bring  the  material  to  proper 
consistency  for  use,  and,  on  account  of  the  variable 
character  of  the  asphalt,  much  care  must  be  taken  in 
handling  it  to  secure  good  results. 

BERMUDEZ   ASPHALT 

Asphalt  obtained  from  the  Bermudez  pitch  lake  in 
Venezuela  is  commonly  known  as  Bermudez  asphalt. 
This  deposit  is  much  greater  in  area  than  that  at  Trinidad, 
being  some  900  acres  in  extent.  It  is,  however,  shallow 
in  depth;  not,  as  in  Trinidad,  in  a  deep  crater,  but 
spread  out  in  a  flat  layer  over  the  surface  of  the  ground 
from  a  number  of  springs,  some  of  which  are  still  active. 
The  depth  of  the  deposit  is  from  two  to  nine  feet  and 
covered  with  vegetation.  The  ground  is  flat  and  swampy, 
so  that  excavations  fill  with  water,  and  in  the  rainy 
season  the  deposits  are  largely  covered  with  water. 

The  asphalt  has  come  from  the  spring  in  a  soft  condition, 
and  afterward  hardened  upon  exposure.  Some  of  these 
springs  are  still  active,  and  are  surrounded  by  small 
areas  of  soft  pitch  raised  above  the  general  level  of  the 
deposit.  The  surface  of  the  deposit  in  general  is  covered 
with  a  crust,  probably  due  to  coking  produced  by  burning 


ASPHALT  PAVEMENTS.  267 

vegetation,  and  this  surface  is  cut  through  to  obtain  the 
asphalt  from  beneath.  These  deposits  have  been  described* 
by  Mr.  Richardson,  who  has  made  a  careful  study  of 
them. 

The  Bermudez  asphalt  is  more  variable  in  character 
than  that  from  Trinidad.  It  contains  very  little  mineral 
matter,  and  the  bitumen  is  softer,  carrying  a  higher  per- 
centage of  malthenes.  The  crude  asphalt  contains  con- 
siderable water  and  organic  impurities,  which  are  removed 
by  refining  as  with  the  Trinidad  asphalts. 

Refined  Bermudez  Asphalt  contains  usually  93  per 
cent  to  97  per  cent  bitumen,  2  per  cent  to  5  per  cent 
of  other  organic  matter,  and  i  per  cent  to  3  per  cent 
inorganic,  or  mineral  matter.  The  bitumen  contains 
64  per  cent  to  72  per  cent  of  malthenes  soluble  in  88° 
naphtha  solution,  and  has  2  per  cent  to  5  per  cent  of 
petrolenes,  volatile  at  325°  F.  in  7  hours.  It  is  therefore 
somewhat  softer  and  more  volatile  than  the  Trinidad 
asphalt. 

Another  deposit  of  asphalt  in  Venezuela  produces 
material  known  as  "  Maracaibo  asphalt "  somewhat 
similar  to  the  Bermudez  asphalt  and  used  for  paving. 

UTAH   ASPHALT 

Considerable  deposits  of  asphaltic  materials  are  found 
in  Utah  and  Colorado.  Of  these,  the  most  important 
are  large  deposits  of  nearly  pure  bitumen  known  as 
Gilsonite.  This  material  carries  no  mineral  matter  and 
is  almost  entirely  soluble  in  carbon  bisulphide.  It  does 
not  therefore  need  refining  or  dehydrating  before  use. 
The  proportion  of  malthenes  is  usually  much  less  than 

*On  the  Nature  and  Origin  of  Asphalt,  Long  Island  City,  1898. 
The  Modern  Asphalt  Pavement,  New  York,  1908. 


268      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

that  of  the  Trinidad  asphalts,  although  there  is  considerable 
variation  in  the  composition  of  the  different  veins.  The 
character  of  the  hydrocarbons  is  also  quite  different 
from  those  of  the  asphalts,  the  malthenes  being  mainly 
composed  of  unsaturated  hydrocarbons.  The  Utah  gil- 
sonite  is  used  to  considerable  extent  for  paving  and  also 
in  the  preparation  of  asphalt  cement  for  filling  the  joints 
in  brick  and  stone  pavements,  as  well  as  asphalt  paints 
and  waterproofing  mixtures. 

CALIFORNIA  ASPHALT 

Several  deposits  of  asphalt  in  Southern  California  have 
been  developed  and  more  or  less  used  for  paving  purposes. 
The  individual  deposits  in  most  cases  are  small  in  extent 
and  somewhat  expensive  to  work.  The  asphalt  usually 
differs  from  those  already  described  in  being  harder,  the 
bitumen  containing  less  malthenes.  Some  of  them  have 
a  higher  percentage  of  mineral  matter  and  less  bitumen 
than  the  Trinidad;  others  have  only  a  small  amount 
of  mineral  matter  but  are  composed  of  a  harder  bitumen. 

Bitumen  similar  to  that  in  the  natural  asphalts  is 
produced  as  a  residual  pitch  in  the  distillation  of  the 
asphaltic  petroleums  of  California.  The  character  of  this 
bitumen  depends  upon  the  extent  to  which  the  distillation 
is  carried  and  the  care  used  in  the  operation.  These 
residues  have  been  used  to  a  considerable  extent  in 
paving  under  the  name  "  D  "  grade  asphalt.  With  proper 
manipulation  of  the  process  the  product  may  be  con- 
trolled and  the  proportions  of  malthenes  and  asphaltenes 
regulated.  Where  the  material  is  overheated  and  burned, 
some  of  the  asphaltenes  are  changed  to  carbenes,  or 
to  insoluble  organic  matter.  Much  care  is  therefore 
required  for  the  preparation  of  good  materials,  and 


ASPHALT  PAVEMENTS.  269 

failures  have  frequently  resulted  from  the  use  of  that 
which  has  been  carelessly  prepared  and  cracked.  It  is 
necessary  that  the  residuum  be  prepared  at  a  low  tem- 
perature to  prevent  cracking.  The  following  specification 
is  given*  by  Richardson  for  "  D  "  grade  asphalt  suitable 
for  use  in  paving: 

" l  D  '  grade  asphalt  should  be  the  residue  from  the 
careful  distillation,  with  steam  agitation,  of  some  suitable 
California  petroleum  at  as  low  temperature  as  possible 
and  certainly  not  exceeding  700°  F.  It  shall  be  free 
from  carbon  or  suspended  insoluble  matter,  which  are 
evidence  of  excessive  cracking. 

"  It  shall  be  soluble  to  the  extent  of  at  least  98  per 
cent  in  carbon  disulphide,  95  per  cent  in  cold  carbon 
tetrachloride  and  not  less  than  65  nor  more  than  80  per 
cent  of  it  shall  be  soluble  in  88°  Pennsylvania  naphtha, 
preferably  nearer  the  former  figure. 

"  It  shall  not  flash  below  450°  F.  and  shall  have  a 
density  between  1.04  and  1.06.  It  shall  not  volatilize 
more  than  8.0  per  cent  at  400°  F.  in  4  hours,  and  shall 
have  a  penetration  between  40°  and  70°.  It  shall  melt 
at  not  less  than  140°  nor  over  180°  F.  on  mercury,  according 
to  the  method  in  use  in  the  New  York  Testing  Laboratory, 
and  shall  yield  not  more  than  15  per  cent  of  fixed  carbon 
on  ignition. 

"  It  shall  have  a  consistency  of  not  less  than  four 
(4)  mm.  penetration  at  78°  F.  when  tested  for  five  (5) 
seconds  with  a  No.  2  needle  weighted  with  100  grams." 

ASPHALTIC   SAND. 

Deposits  of  sand  impregnated  with  asphalt  occur  at  a 
number  of  points  in  California,  Kentucky,  Utah,  and 

*  The  Modern  Asphalt  Pavement,  New  York,  1908. 


270      A   TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

Indian  Territory.  These  deposits  consist  of  sand,  or 
sandstone,  saturated  with  bitumen.  They  differ  from 
each  other  in  the  amount  of  bitumen  found  in  them, 
and  in  the  sand  grains.  They  contain  from  about  5 
per  cent  to  20  per  cent  of  bitumen,  which  in  most  instances 
have  a  larger  percentage  of  malthenes  and  are  much 
softer  than  those  of  the  Trinidad  and  Bermudez  asphalts. 
They  also  frequently  contain  considerable  petrolene,  or 
matter  volatile  at  325°  F.  in  7  hours;  in  some  instances 
as  much  as  10  per  cent  to  12  per  cent.  The  bitumens 
of  these  materials  may  be  classed  as  malthas  rather 
than  asphalts,  and  contain  too  much  volatile  matter  for 
successful  use  as  paving  materials.  They  have  been 
used  to  some  extent  in  paving.  In  California  the  bitumen 
has  been  extracted  from  the  sand  by  the  use  of  naphtha 
and  then  refined  and  used  in  the  same  way  as  the  Venezuela 
asphalts.  The  Kentucky  asphaltic  sandstones  have  been 
used  by  adding  to  them  a  harder  bitumen,  and  finer 
mineral  matter  for  filler.  The  Indian  Territory  material 
has  been  used  for  pavements  by  mixing  it  with  an  asphaltic 
limestone  which  also  occurs  in  the  same  locality. 

ROCK   ASPHALT. 

Limestone  impregnated  with  bitumen  occurs  in  many 
places  in  Europe  and  a  few  localities  in  the  United  States. 
The  rock  is  mined  at  a  number  of  places  in  Europe, 
notably  at  Seyssel,  France;  Travers,  Switzerland;  Ragusa, 
Italy;  and  Verwohle,  Germany.  It  is  usually  composed 
of  nearly  pure  carbonate  of  lime,  impregnated  with  from 
5  per  cent  to  20  per  cent  of  bitumen.  Natural  rock 
asphalt  suitable  for  paving  purposes  usually  contains 
from  9  per  cent  to  20  per  cent  of  bitumen.  The  rock 
should  be  of  fine,  even  grain,  and  have  the  bitumen 


ASPHALT  PAVEMENTS.  271 

uniformly  distributed  through  it.  In  forming  the  surface 
material  for  pavements,  the  rock  from  different  mines 
is  commonly  mixed  in  such  proportions  as  to  give  about 
10  per  cent  to  12  per  cent  of  bitumen  in  the  mixture. 

Deposits  of  bituminous  limestone  exist  in  Texas, 
Indian  Territory,  and  Utah  in  the  United  States.  That 
of  the  Indian  Territory  has,  as  already  stated,  been  used 
for  paving  in  connection  with  asphaltic  sand. 

ART.  67.    ASPHALTIC  CEMENT. 

Refined  asphalt  is  brittle  at  ordinary  temperatures 
and  possesses  little  cementitious  value.  To  bring  it 
to  a  proper  consistency  it  is  heated  to  a  temperature 
of  about  300°  F.  and  mixed  with  heavy  bituminous 
oil,  which  serves  as  flux.  The  product  is  then  known 
as  asphalt  cement. 

Fluxes.  The  material  commonly  used  to  soften  asphalt 
in  preparing  paving  cement  is  the  oil  residuum  resulting 
from  the  distillation  of  petroleum.  These  residuums  are 
prepared  as  mentioned  in  Art.  45,  and  consist  almost 
entirely  of  malthenes  from  which  the  petrolenes  and 
lighter  oils  have  been  removed. 

In  the  preparation- of  the  residuums  for  this  purpose, 
it  is  necessary  that  the  distillation  be  carried  far  enough 
to  remove  all  oils  which  may  be  volatilized  at  the  tem- 
perature to  which  the  material  must  be  subjected  in 
use,  in  order  that  the  consistency  of  the  cement  remain 
constant.  Beyond  this  point  the  extent  to  which  the 
distillation  should  be  carried  depends  upon  the  kind 
of  asphalt  and  the  service  to  which  it  is  to  be  subjected. 
With  dense  and  heavy  oils  a  larger  amount  of  oil  must 
be  used  to  reach  the  same  consistency  than  with  light 
oils,  and  the  stability  of  the  mixture  is  greater  when 


272      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

maintained  in  heated  condition  for  a  long  time,  on  account 
of  the  less  volatile  nature  of  the  oils.  The  character  of 
the  flux  employed  must  be  such  as  to  give  a  proper 
relation  between  the  percentages  of  malthenes  and  asphal- 
tenes  present  in  the  resulting  cement  when  brought  to 
the  required  consistency.  This  is  usually  regulated  in 
practice  by  specifying  the  penetration  to  be  shown  by 
the  cement  as  well  as  the  limits  within  which  the  per- 
centage of  malthenes  to  total  bitumen  may  vary. 

For  Trinidad  and  Bermudez  asphalts,  fluxes  made  from 
either  the  paraffine,  or  the  semi-asphaltic,  petroleums 
have  been  found  quite  satisfactory,  although  the  asphaltic 
oil  when  carefully  prepared  seems  the  more  desirable. 
The  dense  residuums  of  the  California  asphaltic  oils 
have  not  been  found  successful  with  these  asphalts.  The 
amount  of  residuum  required  for  these  asphalts'  varies 
from  about  15  per  cent  to  25  per  cent  of  the  weight  of 
the  asphalt.  If  the  residuum  be  so  dense  as  to  require 
larger  quantities  to  flux  the  asphalt  to  the  required  con- 
sistency, the  percentage  of  malthenes  becomes  too  great 
and  the  cement  is  made  more  susceptible  to  temperature 
changes. 

Asphaltic  materials  like  gilsonite  and  grahamite  con- 
taining less  percentages  of  malthenes  are  fluxed  by  the 
use  of  heavy  asphaltic  residuums,  which  supply  to  the 
cement  the  lacking  malthenes.  Much  larger  proportions 
of  flux  must  be  used  with  these  materials  than  are  required 
for  the  Trinidad  and  Bermudez  asphalts,  the  amount 
varying  with  the  percentage  of  malthenes-  in  the  asphalt. 

Natural  malthas  have  sometimes  been  used  as  fluxes 
for  asphaltic  cement,  but  in  most  instances  difficulty 
has  been  met  in  their  use  due  to  the  fact  that  they  con- 
tain considerable  of  the  lighter  oils,  petrolenes,  which  are 
volatilized  at  the  temperature  required  for  mixing,  thus 


ASPHALT  PAVEMENTS.  273 

leaving  the  maltha  too  hard  to  act  satisfactorily  as  a  flux. 
The  same  difficulty  is  met  in  use  of  carelessly  prepared 
petroluem  residuum,  which  may  lack  uniformity  of  com- 
position, and  contain  both  volatile  oils  and  hard  asphaltenes. 
Preparation  of  Cement.  In  preparing  asphalt  cement 
the  asphalt  is  first  melted  and  raised  to  a  temperature, 
of  about  300°  F.  The  flux  is  then  added  at  a  tempera- 
ture of  150°  to  200°  F.  The  mass  is  then  agitated  with 
jets  of  steam  or  air.  The  agitation  is  continued  from 
4  to  8  hours,  or  until  the  mass  comes  to  a  uniform  and 
homogeneous  condition.  The  refined  asphalt  cement  is 
then  drawn  off.  Careful  and  expert  manipulation  is 
necessary  to  secure  a  uniform  product  of  proper  con- 
sistency. Continued  agitation  with  air  causes  hardening 
of  some  of  the  bitumens  and  volatilizes  some  of  the 
lighter  oils. 

ART.  68.    TESTS  FOR  ASPHALTIC  CEMENT. 

For  the  purpose  of  controlling  the  character  of  surface 
mixtures  to  be  used  upon  asphalt  pavements,  tests  are 
commonly  made  of  the  asphalt  cement,  as  well  as  of  the 
surface  mixture  itself.  In  testing  asphalt  cement  the 
total  amount  of  bitumen  is  usually  determined;  the 
hydrocarbons  composing  the  bitumen  are  separated  into 
their  various  classes,  and  the  consistency  of  the  mixture 
as  well  as  the  effect  of  temperature  upon  the  consistency 
is  examined.  These  tests  are  much  the  same  as  those 
given  in  Art.  48  for  bitumen  for  road  purposes,  but  vary 
in  some  particulars  according  to  the  use  for  which  the 
material  is  intended.  Tests  for  specific  gravity,  fixed 
carbon,  and  melting-point  are  there  given  and  will  not 
be  repeated  here. 

Total   Bitumen.     The    total    bitumen    in    asphalt,    or 


274      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

asphalt  cement,  is  determined  by  testing  its  solubility 
in  carbon  disulphide.  The  following  method  is  recom- 
mended as  standard  by  the  "  Committee  on  Standard 
Test  for  Road  Materials,"  of  the  American  Society  for 
Testing  Materials.* 

"  It  was  decided,  owing  to  the  great  variety  of  con- 
ditions met  in  asphalt  and  like  bitumen,  that  it  was 
impossible  to  specify  any  one  method  of  drying  that 
could  be  at  all  satisfactorily  applied  in  every  case;  it 
is  therefore  supposed  that  the  material  for  analysis  has 
been  previously  dried  either -in  the  laboratory  or  in  the 
process  of  refining  or  manufacture,  and  that  water,  if 
present,  exists  only  as  moisture  in  the  hydroscopic  form. 

"  The  material  to  be  analyzed,  if  sufficiently  hard  and 
brittle,  is  ground  and  then  spread  in  a  thin  layer  in 
a  suitable  dish"  (iron  or  nickel  will  do),  and  kept  at  a 
temperature  of  125°  C.  for  one  hour.  In  the  case  of 
paving  mixture,  where  it  is  not  desirable  to  crush  the 
sand  grains,  a  lump  may  be  placed  in  the  drying  oven 
until  it  is  thoroughly  heated  through,  when  it  can  be 
crushed  down  into  a  thin  layer  and  dried  as  above.  If 
the  material  under  examination  contains  any  hydro- 
carbons at  all  volatile  at  this  temperature,  it  will  of 
course  be  necessary  to  resort  to  other  means  of  drying. 
Tar  or  oils  may  be  dehydrated  by  distillation  and  the 
water-free  distillate  returned  to  the  residue  and  thor- 
oughly incorporated  with  it. 

"  Analysis  of  Sample.  After  drying,  from  2  to  15 
grams  (as  may  be  necessary  to  insure  the  presence  of 
i  to  2  grams  of  pure  bitumen)  is  weighed  into  a  150-0.0. 
tared  Erlenmeyer  flask,  and  treated  with  TOO  c.c.  of 
carbon  disulphide.  The  flask  is  then  loosely  corked 
and  shaken  from  time  to  time  until  all  large  particles 

*  Proceedings,  American  Society  for  Testing  Materials,  Vol.  VI,  1906. 


ASPHALT  PAVEMENTS.  275 

of  the  material  have  been  broken  up.  It  is  then  set 
aside  for  48  hours  to  settle.  The  solution  is  decanted 
into  a  similar  flask  that  has  been  previously  weighed. 
As  much  of  the  solvent  is  poured  off  as  possible  without 
disturbing  the  residue.  The  contents  of  the  first  flask 
are  again  treated  with  fresh  carbon  disulphide,  shaken  as 
before,  and  then  put  away  with  the  second  flask  for 
48  hours  to  settle. 

"  The  liquid  in  the  second  flask  is  then  carefully  decanted 
upon  a  weighed  Gooch  crucible,  3.2  cm.  in  diameter  at 
the  bottom,  fitted  with  an  asbestos  filter,  and  the  contents 
of  the  first  flask  are  similarly  treated.  The  asbestos 
filter  is  made  of  ignited  long-fiber  amphibole,  packed 
in  the  bottom  of  a  Gooch  crucible  to  the  depth  of  not 
over  J  inch.  In  filtering  no  vacuum  is  to  be  used  and 
the  temperature  is  to  be  kept  between  20°  C.  and  25°  C. 
After  passing  the  liquid  contents  of  both  flasks  through 
the  filter,  the  residue  on  the  filter  is  thoroughly  washed, 
and  the  residues  remaining  in  them  are  shaken  with 
more  fresh  carbon  disulphide  and  allowed  to  settle  for 
24  hours,  or  until  it  is  seen  that  a  good  subsidation  has 
taken  place.  The  solvent  in  both  flasks  is  .then  again 
decanted  through  the  filter  and  the  residues  remaining 
in  them  are  washed  until  the  washings  are  practically 
colorless.  All  washings  are  to  be  passed  through  the 
Gooch  crucible. 

"  The  crucible  and  both  flasks  are  then  dried  at  125°  C. 
and  weighed.  The  filtrate  containing  the  bitumen  is 
evaporated,  the  bituminous  residue  burned,  and  the  weight 
of  the  ash  thus  obtained  added  to  that  of  the  residue 
in  the  two  flasks  and  the  crucible.  The  sum  of  these 
weights  deducted  from  the  weight  of  substance  taken 
gives  the  weight  of  soluble  bitumen." 

Bitumen  Soluble  in  Naphtlw.     This  test  is  employed 


276      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

for  the  purpose  of  determining  the  relative  amounts  of 
asphaltenes  and  malthenes  present  in  the  bitumen. 
When  used  in  specifications  it  is  designed  to  insure  a 
proper  relation  between  these  classes  of  hydrocarbons, 
the  object  being  to  avoid  materials  containing  too  great 
percentage  of  asphaltenes,  and  the  use  of  light  oils  as 
fluxes.  The  Committee  of  the  American  Society  for 
Testing  Materials  recommend  that  the  same  method 
be  employed  as  for  obtaining  the  total  bitumen.  They 
also  recommend  that  the  naphtha  used  be  described 
by  giving  the  temperatures  between  which  it  distills  and 
its  specific  gravity.  Naphtha  of  a  density  of  88°  Be. 
at  60°  F.  is  commonly  employed  for  this  purpose. 

Bitumen  Soluble  in  Carbon  Tetrachloride.  Tests  of  the 
solubility  of  solid  bitumens  in  carbon  tetrachloride  are 
made  for  the  purpose  of  determining  the  character  of 
the  bitumen  through  separating  the  carbenes  from  the 
asphaltenes.  This  test  is  commonly  made  in  the  same 
manner  as  is  used  in  determining  the  solubility  in  carbon 
disulphide.  The  following  method  is  given  by  the  Asso- 
ciation for  Standardizing  Paving  Specifications,  in  1911: 

"  Weigh,  off  one  gram  of  material,  cover  with  200  c.c. 
carbon  tetrachloride,  in  Erlenmeyer  flask,  about  4:00 
in  the  afternoon.  Allow  to  stand  over  night  in  dark 
cupboard.  Next  morning,  at  10  o'clock,  Gooch  crucible 
with  felt  is  weighed  and  solution  poured  through.  Wash 
with  carbon  tetrachloride,  dry,  at  100°  C.  and  weigh." 

Heat  Test.  This  test  is  employed  to  determine  the 
amount  of  the  lighter  and  more  volatile  hydrocarbons 
in  the  bitumen.  It  separates  the  petrolenes  from  the 
heavier  hydrocarbons  (malthenes  and  asphaltenes).  The 
test  is  made  by  determining  the  loss  in  weight  suffered 
by  the  material  upon  being  heated  for  a  definite  time 
at  a  constant  temperature.  It  is  designed  to  show  whether 


ASPHALT  PAVEMENTS.  277 

the  asphalt  cement  will  be  materially  changed  by  heat 
in  forming  the  surface  mixture. 

Methods  of  making  the  test  are  given  in  Art.  48. 

Penetration  Test.  The  consistency  of  asphalt  cement 
is  determined  by  measuring  the  penetration  of  a  No.  2 
needle  under  a  standard  weight  (usually  100  grams), 
in  a  given  interval  of  time  (commonly  5  seconds).  The 
tests  must  be  made  at  a  standard  temperature  (usually 
77°  F.)«  Machines  for  making  this  test  have  been 
devised  by  Mr.  A.  W.  Dow*  and  by  Clifford  Richardson.f 

The  Dow  apparatus  consists  of  a  No.  2  needle  in- 
serted in  a  short  brass  rod  which  is  held  in  an  aluminum 
rod  by  a  binding  screw.  The  aluminum  rod  is  secured 
in  a  framework  so  balanced  that  when  it  is  supported  on 
the  point  of  the  needle  the  framework  and  rod  will 
stand  in  an  upright  position,  allowing  the  needle  to 
penetrate  perpendicularly  without  the  aid  of  support. 
The  frame,  aluminum  rod,  and  needle  weigh  50  grams; 
additional  weight,  when  desired,  is  placed  on  the  bot- 
tom of  the  frame.  The  motion  of  the  sliding  part  is 
communicated  by  a  thread  to  an  index  arm  moving 
over  a  graduated  disk. 

"To  make  the  penetration  test,  the  samples  of 
asphalt  cement  contained  in  circular  tins,  along  with 
the  glass  dish,  are  placed  in  a  receptacle  containing 
at  least  5  inches  of  water,  which  should  have  been  pre- 
viously brought  to  the  temperature  at  which  it  is 
desired  to  make  the  test.  While  the  samples  are  under 
the  water  it  should  be  stirred  every  few  minutes,  best 
with  a  thermometer,  and  the  temperature  kept  con- 
stant when  necessary  by  the  addition  of  hot  or  cold 
water  as  the  case  may  require.  The  samples  should 

.     *  Proceedings,  American  Society  for  Testing  Materials,  Vol.  III. 
f  Proceedings,  American  Society  for  Testing  Materials.  Vol.  VH. 


27&       A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

remain  under  water  at  least  15  minutes,  and  in  cases 
where  their  temperature  is  not  near  that  at  which 
the  test  is  made  they  should  be  left  in  possibly  half  an 
hour.  After  the  samples  have  remained  in  the  water 
a  sufficient  time  to  have  attained  its  temperature  they 
are  ready  to  be  penetrated/' 

For  the  purpose  of  determining  the  effect  upon  con- 
sistency of  changes  of  temperature,  tests  are  made  of 
the  penetration  at  different  temperatures.  Mr.  Dow 
recommends  the  following  standards:  "The  needle 
which  I  have  adopted  as  a  standard  for  penetration 
is  a  No.  2,  manufactured  by  R.  J.  Roberts,  Redditch, 
England.  All  the  needles,  however,  obtained  in  a 
package  cannot  be  used  for  penetrating,  as  they  vary 
somewhat  in  shape,  and  only  those  are  selected  wrhich 
give  a  penetration  corresponding  to  the  standard 
needle.  The  standards  that  I  have  adopted  for  this 
machine  are:  At  32°  F.  or  lower,  the  distance  in  one- 
hundredths  of  a  centimeter  that  a  No.  2  needle  will 
penetrate  into  the  sample  in  one  minute  of  time  when 
weighted  with  200  grams.  For  tests  made  at  a  tem- 
perature of  77°  F.,  the  distance  in  one-hundredths  of 
a  centimeter  that  a  No.  2  needle  will  penetrate  into 
the  sample  in .  5  seconds  of  time  when  weighted  with 
100  grams.  For  tests  made  at  a  temperature  of  1 00°  F., 
or  above,  the  distance  in  one-hundredths  of  a  centi- 
meter that  a  No.  2  needle  will  penetrate  in  5  seconds  of 
time  weighted  with  50  grams. 

"  The  following  is  a  table  giving  the  penetration  and 
ductility  of  three  classes  of  asphalt  cement,  which  I 
have  designated  as  A,  B,  and  C: 


ASPHALT  PAVEMENTS. 


279 


A 

B 

C 

Penetration  at  — 

12°  F 

10 

12 

2C 

77°  F                                                           

cc 

47 

4? 

100°  F 

JCQ 

no 

77 

115°  F 

3^0 

220 

I2O 

Ductility  at  77°  F 

300 

7C 

2O 

"  It  has  been  found  from  practical  experience  that 
it  is  not  safe  to  use  an  asphalt  that  is  more  susceptible 
to  changes  in  temperature  than  sample  A,  given  in 
the  table,  for  if  it  were  more  susceptible  than  this, 
and  made  to  a  softness  to  give  sufficient  ductility  at 
low  temperatures,  it  would  be  too  soft  for  use  at  high 
temperatures.  The  average  paving  cement  gives 
penetrations  such  as  represented  by  B  in  the  table. 
Sample  C  in  the  table  represents  the  least  susceptible 
cement  which  I  have  found  on  the  market.  This  non- 
susceptibility  to  change  in  temperature  would  be  of 
great  advantage  if  it  were  not  for  the  fact  that  the 
cement  is  lacking  in  ductility.  There  is  a  law  which 
I  have  found  that  invariably  applies  to  the  proper- 
ties of  asphalt  cements,  that  is,  that  the  less  susceptible 
cement  is  to  change  in  temperature,  the  less  ductile  it 
is  at  normal  temperatures,  and  inversely,  the  more 
susceptible  the  more  ductile  is  the  cement/' 

Test  for  Ductility.  Mr.  A.  W."  Dow  *  has  proposed 
a  test  of  the  ductility  of  asphalt  cement  by  determining 
the  distance  in  centimeters  that  a  prism  of  cement  can 
be  drawn  out  before  breaking.  The  prism  he  uses  is 
5  centimeters  in  length  with  a  square  cross-section  of 
I  centimeter.  The  test  piece  is  molded  with  the  ends 
in  clips,  which  may  be  attached  to  apparatus  for 

*  Proceedings.  American  Society  for  Testing  Materials,  Vol.  III. 


280      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

applying  the  pull.  The  clips  are  pulled  apart  at  a 
speed  of  I  centimeter  per  minute,  while  immersed  in 
water  at  the  required  temperature.  "Sufficient  work 
has  not  been  done  on  the  ductility  test  at  low  temper- 
atures to  be  able  to  state  any  standard  at  the  present 
time,  but  it  has  been  found  that  it  is  not  safe  for  an 
asphalt  having  a  consistency  of  40  penetration  at  77°  F. 
to  pull  less  than  20  centimeters  at  this  temperature  in 
the  above  ductility  test/' 

Impact  Test.  An  impact  test  for  the  purpose  of 
determining  the  toughness  of  asphalt  surface  mixtures 
has  been  proposed  by  Messrs.  Richardson  and  Forrest.* 
"The  test  pieces  were  made  as  follows:  The  surface 
mixture  was  brought  to  such  a  temperature  as  would 
be  found  necessary  in  handling  it  upon  the  street,  a 
weighed  amount,  such  as  has  been  found  by  experi- 
ence would  yield  a  cylinder  after  compression  of  I 
inch  in  height,  is  placed  in  a  cylindrical  mold,  closely 
resembling  the  ordinary  diamond  mortar  of  the  labo- 
ratory, of  a  diameter  of  i£  inch.  The  mold  is  supported 
on  the  rigid  block  of  timber  nj  by  9^  inches  square 
by  32  J  inches  high.  The  warm  steel  plunger  is  placed 
upon  the  top  of  the  hot  mixture,  above  which  is  a 
cylinder  of  steel  weighing  10  pounds,  running  in 
grooved  guides,  which  can  be  allowed  to  fall  upon  it 
from  a  height  of  3  feet.  After  a  few  gentle  taps  to  seat 
the  plunger,  the  weight  is  raised  and  allowed  to  fall 
freely  10  times.  The  cylindrical  mold  is  then  inverted 
and  the  plunger  introduced  at  the  other  end  in  a  space 
left  for  this  purpose  by  a  boss  on  the  base  supporting 
the  mold.  Ten  additional  blows  are  then  given  on 
this  end  of  the  cylinder.  In  this  way  it  has  been  found 
that  satisfactory  and  uniform  compression  is  obtained. 

*  Proceedings,  American  Society'for  Testing  Materials,  Vol.  V. 


ASPHALT  PAVEMENTS.  281 

The  cylinders  are  then  weighted  to  determine  if  the 
density  is  satisfactory,  and  measured  to  see  that  they 
are  of  uniform  height,  I  inch  or  nearly  so.  On  cooling 
they  are  ready  to  be  tested,  in  the  same  manner 
employed  by  Mr.  Page  for  rock  cylinders,  at  whatever 
temperature  may  be  selected"  (see  Art.  37). 

Separation  of  Bitumen.  For  the  purpose  of  testing 
the  bitumen  in  surface  mixtures,  or  in  asphalt  cement 
containing  considerable  mineral  matter,  it  may  be 
necessary  to  separate  the  bitumen  from  the  mixture. 
The  following  method  is  given  by  Mr.  Dow:  *  "The 
pure  bitumen  is  obtained  from  an  asphalt,  or  asphaltic 
cement,  by  extracting  with  carbon  disulphide  and 
evaporating  off  the  solvent.  The  procedure  that  I 
have  found  to  give  the  best  results  is  as  follows :  Suffi- 
cient of  the  asphalt  or  asphaltic  cement  to  give  30 
grams  of  pure  bitumen  is  placed  in  a  large  Erlenmeyer 
flask.  Between  300  and  400  centimeters  of  carbon 
disulphide  is  added,  the  flask  corked  and  then  shaken 
from  time  to  time  until  none  of  the  asphalt  is  seen 
adhering  to  the  sides  or  bottom,  after  which  the  flask 
is  set  aside  and  allowed  to  stand  for  24  hours.  The 
carbon  disulphide  is  then  decanted  off  carefully  from 
the  residue  into  a  second  flask.  The  residue  is  again 
treated  with  200  or  300  cubic  centimeters  of  the 
solvent  and  shaken  as  before.  After  the  solutions 
in  the  two  flasks  have  been  allowed  to  subside  for  24 
hours,  the  contents  are  carefully  decanted  off  on  to  an 
asbestos  filter,  passing  the  contents  of  the  second 
flask  through  the  filter  first.  The  solvent  containing 
the  bitumen  is  then  distilled  in  a  flask  until  just  suffi- 
cient remains  to  have  the  contents  liquid.  It  is  then 
poured  into  a  flat  evaporating  dish  and  further  heated 

*  Proceedings,  American  Society  for  Testing  Materials,  Vol.  III. 


282       A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

on  the  steam-bath,  stirring  from  time  to  time,  until 
the  greater  part  of  the  carbon  disulphide  is  evaporated. 
About  one-half  cubic  centimeter  of  water  is  next 
incorporated  into  the  residue  of  bitumen  and  the  heat- 
ing continued  over  a  burner  until  all  foaming  ceases, 
after  which  it  is  kept  at  300°  F.  for  10  minutes. 
While  heating  over  the  burner  the  bitumen  should  be 
stirred  constantly  with  a  thermometer  and  care  exer- 
cised that  the  temperature  is  kept  constant  at  300°  F. 
It  is  doubtful  whether  in  all  cases  the  last  traces  of 
carbon  disulphide  are  removed,  even  by  this  method, 
and  it  is  also  likely  that  the  bitumen  obtained  in  this 
way  is  often  slightly  harder  than  that  contained  in 
the  original  asphalt  or  cement;  but  its  physical  prop- 
erties, as  far  as  ductility  and  susceptibility  to  change 
in  temperature  go,  will  be  relatively  the  same,  and  a 
sufficiently  close  approximation  can  be  made  of  the 
consistency  of  the  bitumen  in  the  original  sample  to 
answer  all  practical  purposes.  As  the  removal  of  the 
last  traces  of  carbon  disulphide  is  very  difficult,  and  a 
soft  bitumen  is  liable  to  be  hardened  in  so  doing,  I 
make  it  a  practice,  wherever  it  is  possible,  to  extract 
the  bitumen  from  an  asphalt  before  it  has  been  soft- 
ened into  the  paving  cement.  In  this  way  I  find  it 
easier  to  remove  the  last  traces  of  solvent  from  this 
.hard  bitumen,  and  at  the  same  time  with  relatively 
less  hardening.  This  bitumen  from  the  asphalt  is 
then  fluxed  into  a  paving  cement  by  adding  to  it  an 
amount  of  flux  equivalent  to  that  used  in  making  the 
paving  cement  from  the  asphalt.  It  is  fortunate  that 
nearly  all  the  asphalts  met  with  in  commerce  that  are 
not  pure  bitumen  are  of  a  hard  nature,  so  that  the 
above  method  is  applicable  in  practically  all  cases. 
This  of  course  does  not  apply  to  bituminous  rock/  and 


ASPHALT   PAVEMENTS. 


283 


the  only  way  possible  to  estimate  their  quality  is  by 
examining  the  extracted  bitumen,  which  is  done  as 
just  described.  It  is  well  to  note  here  that  in  cases 
where  the  bitumen  hardens  materially  in  the  removal 
of  the  solvent,  such  a  bitumen  will  be  rejected  by 
hardening  too  much  in  the  heat  test." 

Examination  of  Mineral  Aggregates.  For  the  purpose 
of  determining  the  character  of  the  mineral  aggregate 
used  in  a  surface  mixture,  or  present  in  an  asphaltic 
cement,  the  American  Society  for  Testing  Materials 
recommends*  that  it  be  passed  through  sieves  of  the 
following  sizes  in  the  order  named. 


Meshes  per 

Diametei 

•  of  Wire. 

Linear  Inch. 

Inches. 

Mm. 

2OO  
IOO  
80  
50  

0.00235 
0.0045 
0.00575 
O.OOQ 

0.05969 
0.1143 
o.  1460 
0.22865; 

4.O 

o  oio/'5 

o  2603^ 

3O 

o  0137? 

O    34.Q  2  <C 

2O  
IO  

0.0165 
0.027 

0.4191 
0.6858 

Tests  Required  by  Specifications.  Many  tests  have  been 
proposed  for  the  control  of  asphalt  paving  mixtures,  or 
used  in  the  study  of  asphalt  materials,  by  various  inves- 
tigators. In  general,  however,  specifications  used  by 
municipal  engineers  have  depended  upon  a  contractor's 
guaranty  for  the  character  of  the  work  rather  than  upon 
inspection  of  the  materials  and  workmanship.  This  has 
not  proven  altogether  satisfactory  and  it  is  now  common 
to  have  definite  specifications  for  the  materials  employed 
and  subject  them  to  test  in  the  city  laboratories. 


284      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  69.     SURFACE  MIXTURES. 

The  material  commonly  emploj^ed  for  the  surfaces 
of  asphalt  pavements  consists  of  a  mixture  of  asphalt 
cement,  powdered  limestone,  and  sand.  The  mixtures 
used  in  different  places  have  varied  considerably  in 
character,  according  to  the  nature  of  the  materials 
available  and  the  amount  and  consistency  of  the 
bitumen  employed.  Experience  has  gradually  devel- 
oped the  practice  in  different  places,  the  work  at  first 
being  largely  experimental,  defects  in  the  early  work 
being  corrected  by  modifications  in  later  mixtures. 
Much  of  this  work  has  been  done  by  very  haphazard 
methods  and  without  any  careful  analysis  of  the 
causes  of  defects  and  failures,  or  of  the  differences  in 
materials  used  in  different  places. 

Sand.  It  is  common  to  grade  sand  in  size  by 
sifting  through  sieves  of  10,  20,  30,  40,  50,  80,  100  and 
200  meshes  to  the  linear  inch,  finding  the  percentage 
which  passes  each  sieve  and  is  caught  by  the  next 
finer  one.  The  portion  which  passes  the  200-mesh 
sieve  is  too  fine  to  be  considered  as  sand,  and  is  classed 
with  the  stone  powder  which  is  added  as  filler.  The 
sands  used  for  asphalt  surface  mixtures  are  much 
finer  than  those  employed  in  cement  mortars.  In 
sand  for  this  purpose  most  of  the  sand  is  usually  fine 
enough  to  pass  the  40-mesh  sieve.  In  some  instances, 
the  bulk  of  the  sand  will  pass  through  the  80  and  1 00- 
mesh  sieves;  in  others,  the  larger  portion  will  only  pass 
the  40  and  50-mesh  sieves.  A  grading  of  sizes  in  the 
sand  is  desirable  on  account  of  reducing  the  amount 
of  voids  to  be  filled  by  the  asphalt,  and  frequently 
when  a  natural  sand  of  correct  sizes  is  not  available, 
it  is  possible  to  secure  a  proper  relation  of  sizes  of 


ASPHALT  PAVEMENTS.  285 

grain  by  mixing  two  or  more  sands  of  differing  sizes. 
In  general,  sand  so  graduated  as  to  leave  a  small 
percentage  of  voids  is  desirable  in  order  that  the 
interstices  may  be  fully  filled  with  bitumen,  but  the 
sand  of  greatest  density  is  not  necessarily  the  best 
for  this  purpose,  as  there  may  be  instances  where  the 
percentage  of  voids  would  not  admit  of  a  sufficient 
amount  of  bitumen. 

The  voids  in  the  sand  are  commonly  tested  by 
rilling  a  measure  with  packed  sand  and  then  deter- 
mining the  quantity  of  water  that  can  be  added  to  it. 
A  better  method  with  fine  material  is  to  determine 
the  specific  gravity  of  the  sand  and  also  the  weight  of 
a  given  volume  of  it,  the  volume  of  voids  being  the 
difference  between  the  measured  volume  and  the 
volume  of  solid  matter  represented  by  its  weight. 

The  sand  used  should  be  hard  and  tough  in  order  to 
resist  wear  well,  but  its  chemical  character  is  not  of 
special  importance,  although  there  seems  to  be  a 
difference  in  the  adherence  of  bitumen  to  the  surfaces 
of  different  sands.  The  reason  for  this  difference  is 
not  apparent  and  cannot  be  judged  in  advance  of  actual 
trial.  The  shape  of  grain  does  not  usually  appear 
important.  Rounded  grains  often  pack  more  easily 
and  form  a  more  dense  mass,  but  it  has  been  sometimes 
thought  that  they  move  more  readily  upon  each  other 
and  form  a  less  firm  surface. 

Filler.  The  filler  used  in  asphalt  paving  mixtures 
consists  of  very  finely  ground  mineral  matter  mixed 
with  the  bitumen  for  the  purpose  of  rendering  the 
surface  more  dense,  and  giving  stiffness  to  it.  The 
material  commonly  used  for  this  purpose  is  ground 
limestone,  although  a  number  of  other  materials  have 
been  emplo37ed.  Ground  cla}'  may  make  a  good 


286      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

filler;  slaked  lime,  and  Portland  or  natural  cement 
are  also  used,  especially  good  results  being  obtained 
with  Portland  cement. 

The  filler  should  be  finely  ground;  nearly  all  of  it  is 
usually  required  to  pass  a  200-mesh  sieve,  but  the 
finer  portions  are  too  fine  to  be  graded  by  the  use  of 
sieves.  For  this  purpose  the  method  of  elutriation 
may  be  employed.  By  this  method  the  powders  of 
different  degrees  of  pulverization  are  separated  by 
observing  the  times  required  to  settle  after  being  shaken 
in  a  vessel  of  distilled  water,  the  portions  which  settle 
in  15  seconds,  one  minute,  and  30  minutes  being  deter- 
mined, and  the  relative  fineness  of  different  samples 
thus  compared. 

Composition  of  Mixture.  The  relative  amounts  of 
asphalt  cement  filler,  and  sand  required  for  a  surface 
mixture  must  of  course  depend  upon  the  properties  of 
these  materials.  The  fineness  of  the  sand  and  of  the 
filler,  and  the  amount  of  mineral  matter  in  the  cement, 
are  all  important  in  determining  the  proper  propor- 
tions. The  proportions  of  materials  are  determined 
by  weight,  the  purpose  being  to  secure  a  proper 
amount  of  bitumen  and  of  dust,  as  compared  with 
the  sand  in  the  resulting  mixture.  The  amount  of 
bitumen  required  varies  from  about  9  per  cent  to  13 
per  cent,  most  commonly  between  10  per  cent  and  1 1 
per  cent.  Mr.  Richardson  gives*  the  following  mixture 
as  a  standard  to  be  used  for  surfaces  of  Trinidad 
asphalt  pavements : 

*  The  Modern  Asphale  Pavement,  N.  Y.,  1905. 


ASPHALT   PAVEMENTS. 


287 


Surface  Mixture 

Sand. 

Bitumen 

Per  cent 
jO    C 

Per  cent. 

Passing  — 
2oo-mesh  sieve              

I  7    O 

loo-me^h  sieve      

I  2     O 

17  .O 

8o-mesh  sieve 

T  •}      O 

1  7  O 

^o-mesh  sieve 

22     C 

20   O 

4o-mec>h  sieve                  

II    O 

11  .O 

30-me^h  sieve    

8  o 

IO    O 

2o-mesh  sieve    

C    O 

8  o 

lo-mesh  sieve    

3-o 

5-° 

100  .0 

100  .0 

This  he  regards  as  an  exceptionally  good  mixture. 
As  the  sands  used  in  practice  vary  in  the  proportion 
of  fine  grain  which  they  contain,  the  amount  of 
bitumen  must  be  correspondingly  varied.  When  a 
larger  portion  of  the  sand  passes  the  80  and  100-mesh 
sieves,  a  larger  amount  of  bitumen  and  of  filler  may  be 
introduced.  When  the  sand  is  coarser,  a  smaller 
amount  of  bitumen  is  necessary  in  order  that  the 
pavement  may  not  be  soft  enough  to  mark  under  the 
horses'  feet.  In  sand  which  lacks  the  finer  grains 
the  percentage  of  bitumen  which  can  be  used  without 
marking  is  often  so  low  as  to  leave  the  material  too 
porous  and  liable  to  the  action  of  water.  It  is  desir- 
able that  the  mixture  contain  all  the  bitumen  that  it 
will  carry  by  the  addition  of  filler  without  becoming 
too  soft.  A  lack  of  bitumen  may  cause  cracking  of 
the  surface.  The  quantity  of  bitumen  required  is 
also  somewhat  affected  by  the  character  of  the  sand 
grains  and  the  extent  to  which  the  bitumen  may  adhere 
to  and  coat  the  grains.  Some  sands  will  "take* 
more  bitumen  than  others  of  the  same  grading  of  sizes 


288      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

without  leaving  a  surplus  of  bitumen  to  render  the 
material  too  soft. 

The  amount  of  bitumen  to  be  used  in  a  surface  mix- 
ture is  commonly  tested  by  the  pat  test.  This  consists 
in  pressing  a  pat  of  the  surface  material  in  a  piece  of 
brown  manila  paper  and  observing  the  stain  left  upon 
the  paper;  the  depth  of  the  stain  indicates  to  the 
experienced  eye  whether  the  right  amount  of  bitumen 
has  been  used  and  whether  the  mixture  has  been  prop- 
erly prepared.  An  impact  test  is  also  sometimes  made 
to  determine  the  resistance  of  the  surface  material  to 
marking,  and  frequent  analyses  are  made  to  test  the 
correctness  of  the  mixture. 

The  traffic  to  which  a  street  is  subjected  has  much 
to  do  with  the  consistency  required  in  the  surface 
mixture.  For  streets  of  light  traffic  a  softer  mixture 
should  be  employed  than  for  one  with  heavy  traffic. 
The  rolling  out  and  working  of  the  surface  by  heavy 
traffic  will  admit  of  a  hard  surface  material  which  might 
crack  under  light  traffic.  The  surface  mixtures  must 
in  every  case  be  suited  to  the  local  conditions  of  traffic 
and  weather,  that  it  may  neither  mark  under  the 
impact  of  traffic  nor  crack  from  shrinkage  in  cold 
weather. 

Method  of  Mixing.  In  the  preparation  of  the  surface 
mixture,  the  sand  and  asphalt  cement  are  heated 
separately  and  then  mixed  while  hot.  When  two  or 
more  sands  are  used  to  obtain  the  proper  grading  of 
sizes,  this  mixing  must  first  be  accomplished,  and  great 
care  is  necessary  in  handling  the  sand  in  mixer  and 
heater  to  prevent  the  segregation  of  sizes  and  bring  the 
sand  in  uniform  mixture  at  proper  temperature  (about 
330°  to  350°  F.)  to  the  final  mixture.  The  asphalt 
cement  is  also  heated  in  a  large  heater  where  it  is 


ASPHALT  PAVEMENTS.  289 

agitated  by  steam  jets  to  maintain  the  uniformity  of 
mixture. 

The  surface  mixture  is  prepared  in  a  mixer  of  small 
size  which  mixes  10  to  15  cubic  feet  at  one  operation, 
and  is  so  arranged  as  to  load  directly  into  the  wagon 
which  takes  it  to  the  street.  The  mixing  is  accom- 
plished by  blades  revolving  on  shafts  in  the  mixing 
tanks,  requiring  about  one  to  two  minutes  to  make  a 
complete  mixture.  The  proportioning  of  the  ingredi- 
ents is  accomplished  by  weighing  the  proper  quantity 
of  each  of  the  materials  for  a  batch;  the  sand  and  filler 
are  first  introduced  and  mixed  dry,  and  the  asphalt 
cement  then  added  and  the  whole  mixed  together. 
The  mixture  is  then  carried  to  the  street  at  a  tempera- 
ture above  300°  F. 

Specification  Requirements.  The  Association  for  Stand- 
ardizing Paving  Specifications  has  recommended  the 
following  specification  for  the  composition  of  surface 
mixture  for  an  asphalt  pavement: 

"  The  surface  mixture  shall  consist  of  asphaltic  cement, 
Portland  cement  (or  stone  dust),  and  sand  proportioned 
by  weight  so  that  the  resulting  mixture  will  contain  average 
proportions  of  the  whole  mixture  as  follows : 

MIXTURE  A.  Per  Cent. 

Bitumen  soluble  in  cold  carbon  disulphide. .  .  1 1 .  o  to  13 . 5 

Portland  cement  passing  a  No.  200  sieve 10 .  o  to  1 5 .  o 

Sand  passing  a  No.  80  sieve 18 .  o  to  36 .  o 

Sand  passing  a  No.  40  sieve 20 .  o  to  50 .  o 

Sand  passing  a  No.  10  sieve 8 .  o  to  25 .  o 

Sand  passing  a  No.    4  sieve up    to  10.0 

MIXTURE  B. 

Bitumen  soluble  in  cold  carbon  disulphide. .  .  10 . 5  to  13  . 5 

Stone  dust  passing  a  No.  200  sieve 10.0  to  15  .o 

Sand  passing  a  No.  80  sieve 18.0  to  36.0 

Sand  passing  a  No.  40  sieve 20 .  o  to  50 .  o 

Sand  passing  a  No.  10  sieve 8 .o  to  25 . o 

Sand  passing  a  No.    4  sieve up    to  10 .  o 

Sieves  to  be  used  in  the  order  named. 


2QO       A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

"  The  item  designated  '  Portland  cement  (stone  dust) 
passing  a  No.  200  sieve  '  within  the  limits  named  herein 
includes  in  addition  to  the  Portland  cement  (stone  dust) 
fine  sand  passing  a  No.  200  sieve  not  exceeding  4^  per 
cent  of  the  total  mixture,  and  such  2oo-mesh  mineral 
dust  naturally  self-contained  in  the  refined  asphalt. 

"  Sand  and  asphaltic  cement  shall  be  heated  separately 
to  about  300°  F.  The  maximum  temperature  of  the 
sand  at  the  mixers  shall  in  no  case  be  in  excess  of  375° 
F.  at  the  discharge  pipe.  The  Portland  cement  (or  stone 
dust)  shall  be  mixed  with  the  hot  sand  in  the  required 
proportions,  and  then  these  shall  be  mixed  for  at  least 
one  minute  with  the  asphaltic  cement  at  the  required 
temperature,  and  in  the  proper  proportions  in  a  suitable 
apparatus  so  as  to  effect  a  thoroughly  homogeneous 
mixture. 

"  The  proportions  of  asphaltic  cement  shall  at  all 
times  be  determined  by  actual  wieghing  with  scales 
attached  to  the  asphaltic  cement  bucket. 

"  The  Portland  cement  (or  stone  dust)  and  sand  must 
also  be  weighed  unless  a  method  of  gauging  approved 
by  the  (authorized  city  official)  shall  be  used. 

"  The  contractor  shall  furnish  every  facility  for  the 
verification  of  all  scales  or  measures. 

"  The  sand  gradings  and  bitumen  may  be  varied  within 
the  limits  designated,  in  the  discretion  of  (proper  city 
official)." 

Rock  Asphalt.  The  preparation  of  surface  material 
with  rock  asphalt  consists  only  in  crushing  and  grinding 
the  rock  to  powder,  and  heating  the  powder  to  drive 
off  the  water  and  soften  the  bitumen,  so  that  it  may 
be  compacted  in  the  pavement.  The  powder  is  heated 
to  a  temperature  of  200°  to  300°  F.  and  is  applied  hot 
in  laying  the  surface. 


ASPHALT   PAVEMENTS.  291 

Rock  asphalt  as  it  occurs  in  nature  varies  widely  in 
character,  and  seldom  has  a  proper  composition  for  use 
in  pavements  without  admixture  of  other  materials.  In 
determining  the  suitability  of  a  material  of  this  class  for 
use,  the  character  as  well  as  the  quantity  of  bitumen 
contained  by  it  must  be  considered,  and  its  deficiencies 
supplied  in  the  materials  added  to  it.  This  may  some- 
times be  accomplished  by  mixing  different  grades  of  the 
asphaltic  rock,  or  in  other  instances  by  adding  other 
bitumens,  or  mineral  matter. 

In  determing  a  mixture  of  asphalt  rock,  as  in  the  case 
of  other  asphalts,  the  local  conditions  of  climate  and 
traffic  must  be  considered  and  the  quantity  of  bitumen 
be  so  proportioned  as  to  remain  solid  in  summer  and 
not  become  brittle  and  lose  cohesion  in  winter.  The 
surface  mixture  for  a  rock  asphalt  pavement  should 
ordinarily  contain  from  9  to  12  per  cent  bitumen  The 
character  of  bitumen  required  is  about  the  same  as  for 
a -pavement  made  by  mixing  other  asphalts  with  mineral 
matter. 

ART.  70.    CONSTRUCTION  OF  SHEET  ASPHALT  PAVEMENTS. 

The  work  to  be  performed  in  laying  a  sheet  asphalt 
pavement  consists  in  grading  and  rolling  the  road-bed, 
placing  the  foundation,  laying  a  binder  course  of  bitu- 
minous concrete,  and  distributing  and  rolling  the  surface 
material  so  as  to  form  a  smooth  surface.  Sometimes  the 
binder  course  is  omitted,  a  thicker  surface  layer  being 
employed  to  give  sufficient  stiffness  and  prevent  the  sur- 
face scaling  from  the  foundation. 


2Q2      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 


FOUNDATION. 

Concrete  base.  As  a  sheet  asphalt  surface  has  no 
power  to  sustain  loads,  acting  only  as  a  wearing  sur- 
face, which  must  be  held  in  place  from  below,  it  is 
essential  that  it  be  placed  upon  a  very  firm,  unyielding 
foundation.  It  is  consequently  nearly  always  placed 
upon  a  concrete  base,  which  is  commonly  formed  of 
hydraulic  cement  mortar  and  broken  stone,  prepared 
as  described  in  Art.  57.  In  the  use  of  this  base,  it  is 
necessary  that  the  mortar  be  fully  set,  and  the  concrete 
thoroughly  dry  before  the  asphalt  is  laid  upon  it,  as 
the  placing  of  the  hot  surface  material  upon  a  damp 
foundation  wrill  cause  the  blistering  and  possible  dis- 
integration of  the  surface  by  the  steam  generated  from 
the  base  by  the  heat  of  the  material. 

For  moderate  or  heavy  traffic  in  cities,  the  concrete 
base  is  commonly  made  6  inches  thick.  For  lighter 
traffic  a  less  depth,  4  inches  or  5  inches,  is  sometimes 
employed.  The  depth  necessary  will  depend  upon  the 
nature  of  the  road-bed  as  well  as  the  weight  of  the 
traffic.  It  should  be  greater  as  the  subsoil  is  less  firm 
and  well  drained. 

Bituminous  base.  Sometimes  a  base  has  been  used 
consisting  of  a  layer  of  broken  stone  four  or  six  inches 
thick  rolled  into  place  and  coated  with  asphalt  or  coal 
tar  paving  cement.  This  is  known  as  a  bituminous 
base.  The  advantage  which  has  been  claimed  for  it  is 
that  the  foundation  and  surface  material  become 
joined  into  a  single  mass,  with  the  effect  of  anchoring 
the  surface  and  preventing  the  formation  of  weathering 
cracks  and  wave  surfaces,  which  are  sometimes  found 
when  the  hydraulic  base  and  light  surface  layer  are 
employed.  The  hydraulic  base  is  commonly  preferred 


ASPHALT   PAVEMENTS.  293 

to  the  bituminous  base,  which  is  practically  obsolete, 
because  it  forms  an  unyielding  structure,  not  likely  to 
be  forced  out  of  place  by  the  weight  of  traffic  at  any 
point  where  the  support  of  the  road-bed  may  be 
weakened. 

Macadam  base.  In  surfacing  streets  with  asphalt 
which  have  previously  been  macadamized,  it  is  some- 
times possible  to  use  the  old  macadam  as  a  base  for  the 
asphalt.  This  offers  a  good  base  in  so  far  as  it  can  be 
used  without  disturbance.  It  is  difficult,  however,  to 
change  the  grade  or  reduce  the  crown  without  destroy- 
ing the  bond  of  the  macadam.  Old  brick  and  stone 
pavements  may  also  be  used  in  the  same  way.,  where 
they  can  be  used  without  disturbing  them. 

BINDER   COURSE. 

An  intermediate  layer  known  as  the  binder  course  is 
now  commonly  placed  between  the  base  and  surface 
layer.  This  la}Ter  is  ordinarily  about  i^  inches  thick 
and  consists  of  broken  stone,  which  passes  through  a 
I  inch  screen,  mixed  with  sufficient  bitumen  to  thor- 
oughly coat  the  pieces  of  stone.  The  paving  cement 
used  in  making  the  binder  course  should  be  of  softer 
consistency  than  that  used  in  the  surface  material, 
about  3  per  cent  of  bitumen  being  usually  required. 
The  materials  are  mixed  hot,  laid  and  rolled  in  the 
same  manner  as  the  surface  layer.  This  binder  becomes 
consolidated  with  and  gives  added  depth  and  strength 
to  the  surface,  thus  preventing  the  cracks  and  wave 
surfaces  which  may  otherwise  appear.  The  binder,  as 
commonly  formed  of  broken  stone,  is  open  and  porous, 
but  in  some  instances  stone  of  graded  sizes  and  sand 
are  employed  to  make  a  dense  bituminous  concrete. 


294       A   TEXT-BOOK   ON  ROADS   AND   PAVEMENTS. 

This  is  desirable  practice,  adding  materially  to  the 
strength  of  the  pavement  under  heavy  traffic.  It 
requires  a  larger  amount  of  bitumen  (about  5  per  cent 
to  6  per  cent)  on  account  of  the  larger  surface  area  of 
grains  to  be  coated. 

The  binder  course  has,  in  some  instances,  been 
replaced  by  a  coating  of  asphalt  paint,  consisting  of 
asphalt  cement  dissolved  in  benzene.  The  surface  of 
the  hydraulic  base  is  painted  with  this  mixture,  which 
serves  to  cement  the  base  to  the  surface  layer. 

After  the  completion  of  the  hydraulic  base  and  when 
it  has  stood  a  sufficient  length  of  time  to  harden  and 
dry  out,  the  binder  course  is  placed  and  compacted. 
The  binder  is  spread  to  uniform  thickness  over  the 
base  by  use  of  shovels,  all  of  the  material  being  shoveled 
over  in  order  to  secure  uniform  compactness.  It  is 
then  smoothed  with  rakes  having  long  tines,  and  after 
partially  cooling  rolled  with  a  5  or  6  ton  roller. 


SURFACE    COURSE. 

Transportation.  The  materials  for  the  binder  and 
surface  of  asphalt  pavements  must  be  carried  from  the 
mixing  plant  to  the  street  in  some  form  of  truck  or 
wagon  which  will  admit  of  the  materials  being  delivered 
with  small  loss  of  temperature.  Some  form  of  dump 
wagon  is  commonly  employed  for  this  purpose,  carrying 
from  2  to  4  tons  of  the  materials  at  a  load.  The  loss 
of  heat  is  not  rapid  when  the  material  is  carefully 
handled  and  properly  protected  by  tarpaulins,  and  the 
temperature  of  the  mass  should  not  be  reduced  more 
than  about  10  degrees,  where  transportation  to  the 
street  takes  2  or  3  hours. 

Placing.     As  soon  as  the  rolling  of  the  binder  course 


ASPHALT  PAVEMENTS.  295 

has  been  completed,  it  is  ready  for  the  surface  layer. 
This  is  usually  ij  to  IT  inches  thick  where  a  binder 
course  is  used,  or  2  to  2^  inches  in  single  course  work. 
The  surface  material  is  distributed  by  hot  shovels  from 
the  piles  into  which  it  is  dumped  from  the  wagons,  all 
the  material  being  handled  over  as  in  the  case  of  the 
binder.  It  is  then  spread  into  a  smooth  layer  of  proper 
thickness,  with  hot  rakes,  all  lumps  being  broken  and 
the  material  loosened  up  so  that  under  the  roller  it 
may  compact  to  a  uniform  density.  After  raking 
smooth,  the  surface  is  rolled  with  a  steam  roller.  A 
light  roller  (2  to  4  tons)  is  commonly  used  for  the  first 
rolling  until  the  material  is  sufficiently  compact  to 
bear  the  heavier  one  (usually  weighing  6  to  8  tons), 
which  completes  the  shaping  of  the  pavement.  A 
coating  of  dust,  usually  hydraulic  cement,  is  given  to 
the  surface  before  the  final  rolling.  This  gives  proper 
color  to  the  surface. 

The  handling  of  the  material  necessarily  varies  some- 
what with  its  character  and  requires,  for  good  results, 
skill  and  experience  on  the  part  of  the  men  in  charge 
of  the  work.  It  is  highly  important  that  the  material 
be  so  evenly  distributed  as  to  give  a  surface  of  uniform 
density;  otherwise  the  surface  may  compress  unequally 
under  the  traffic,  becoming  uneven  and  wavy.  It  is 
also  necessary  that  the  rolling  be  carefully  done  in 
order  to  properly  compress  the  asphalt  and  bring  the 
surface  to  the  required  form.  When  the  surface  is 
rolled  out  of  shape  through  careless  handling,  it  is 
difficult  to  bring  it  back  again.  The  roller  must  be  so 
balanced  as  to  distribute  the  weight  uniformly,  a 
pressure  of  200  to  300  pounds  per  linear  inch  of  tire 
being  required  for  the  ultimate  compression  of  the 
asphalt  surface. 


296      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Rock  asphalt.  Pavements  of  rock  asphalt  are  con- 
structed in  the  same  manner  as  those  from  free  bitumen. 
The  rock  asphalt  makes  a  harder  surface  and  is  more 
slippery  than  that  made  from  free  bitumen.  It  has 
never  come  so  extensively  into  use  in  the  United 
States.  In  Europe,  where  rock  asphalt  is  very  exten- 
sively used,  pavements  made  from  free  bitumens  mixed 
with  sand  are  frequently  denominated  artificial  asphalt 
as  distinguished  from  asphalt  or  natural  asphalt,  by 
which  is  meant  the  rock  found  impregnated  with 
bitumen. 

In  the  European  rock  asphalt  pavements  the  binder 
course  is  not  so  commonly  employed  as  in  the  United 
States,  and  in  many  cases  the  finishing  of  pavement  is 
by  means  of  tampers  and  smoothing  irons  instead  of 
rollers,  the  compression  given  to  the  surface  not  being 
so  great,  ultimate  compacting  being  accomplished  by 
the  traffic.  At  the  edges  of  the  pavement  and  in  places 
which  cannot  be  reached  by  the  roller,  small  hand  tools 
such  as  hot  smoothing  irons  and  tampers  are  employed 
for  finishing  the  surface.  Sometimes  also  where  the 
rolling  has  failed  to  compress  the  pavement  into  proper 
surface,  it  may  be  necessary  to  soften  the  surface  with 
smoothing  irons  in  order  to  reduce  it  to  the  required 
form. 

ART.  71.     ASPHALT  BLOCKS. 

Asphalt  paving  blocks  are  frequently  formed  of  a 
mixture  of  asphalt  cement  and  crushed  stone.  The 
stone  used  is  mainly  trap,  or  granite,  broken  so  as  to 
pass  a  i  inch  screen.  In  early  work,  limestone  was 
used;  this  was  found  to  lack  durability  on  account  of 
the  softness  of  the  stone.  The  mixture  is  similar  to 


ASPHALT  PAVEMENTS.  297 

that  used  for  the  surface  of  a  sheet  pavement,  containing 
about  8  per  cent  to  1 1  per  cent  of  asphalt  cement, 
7  per  cent  to  10  per  cent  limestone  dust,  and  crushed 
stone  80  per  cent  to  85  per  cent. 

The  materials  are  heated  to  a  temperature  of  about 
300°  F.,  and  mixed  while  hot  in  an  apparatus  arranged 
to  secure  the  even  distribution  of  the  ingredients 
through  the  mass.  The  thorough  incorporation  of  the 
various  materials  in  the  mixture  is  of  first  importance 
in  producing  homogeneous  and  uniform  blocks,  while 
the  quality  of  the  materials  used  needs  as  careful 
inspection  as  in  the  case  of  the  surface  material  for 
sheet  pavements. 

When  the  mixing  is  complete,  the  material  is  placed 
in  moulds  and  subjected  to  heavy  pressure,  after  which 
the  blocks  are  cooled  suddenly  by  plunging  into  cold 
water. 

These  blocks  have  usually  been  made  larger  than 
paving-bricks,  the  common  size  being  12  inches  long, 
3  or  4  inches  wide,  and  4  or  5  inches  deep.  They  are 
laid  in  the  same  manner  as  brick,  as  closely  in  contact 
as  possible,  and  driven  together.  Under  the  action  of 
the  sun  and  the  traffic,  the  asphalt  blocks  soon  become 
cemented  together  through  the  medium  of  the  asphaltic 
cement,  and  form,  like  the  sheet  asphalt  pavements,  a 
practically  impervious  surface.  They  are  often  laid 
upon  gravel  base,  although  in  the  best  work  a  light 
concrete  foundation  is  employed. 

In  forming  the  asphalt  block  pavement  the  road-bed 
is  brought  to  subgrade  in  the  ordinary  manner  and 
rolled,  leaving  room  for  the  pavement  of  uniform  thick- 
ness to  be  placed  upon  it.  A  layer  of  gravel  4  or  5 
inches  deep  is  then  placed  and  rolled,  or  a  base  of  con- 
crete is  formed,  with  a  cushion  coat  of  sand  I  to  2 


298      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

inches,  and  then  the  paving  blocks.  The  blocks  are 
pressed  together  in  the  courses  by  the  use  of  a  lever, 
and  the  courses  driven  against  each  other  with  a  maul 
to  reduce  the  joints  as  much  as  possible.  A  coating 
of  sand  is  given  to  the  surface  of  the  pavement,  and  it 
is  rammed  to  a  firm  and  uniform  surface,  as  in  the  case 
of  brick. 

These  blocks  have  the  advantage  over  sheet  asphalt 
for  the  smaller  cities,  that  the  blocks  may  be  formed 
at  a  central  point  and  shipped  ready  for  use  to  the  site 
of  the  proposed  pavement,  and  that  no  special  plant 
need  be  erected  in  each  town  where  they  are  to  be 
constructed.  They  have  given  satisfaction  in  use,  and 
have  frequently  shown  good  durability  in  wear  under 
moderate  traffic.  It  is  claimed  that  they  are  less 
slippery  and  may  be  used  upon  steeper  slopes  than 
sheet  asphalt.  The  cost  of  transportation  of  the 
blocks  makes  this  pavement  expensive  in  many  locali- 
ties not  in  close  proximity  to  the  place  of  manufacture, 
and  prevents  them  from  competing  successfully  with 
other  pavements. 

ART.  72.     MAINTENANCE  OF  ASPHALT  PAVEMENTS. 

To  give  good  service  asphalt  pavements  must  be 
kept  clean.  On  account  of  the  smooth  surface  and 
absence  of  joints,  cleaning  may  be  readily  accomplished; 
and  the  presence  of  dirt,  especially  in  wet  weather 
when  it  is  likely  to  cause  the  surface  to  remain  damp,  is 
liable  to  cause  the  asphalt  to  rot.  More  than  any 
other  pavement,  therefore,  the  durability  and  wear  of 
an  asphalt  surface  depends  upon  its  cleanliness.  The 
presence  of  dirt  upon  asphalt  in  damp  weather  is  also 
important  in  its  effect  upon  the  slipperiness  of  the 
pavement. 


ASPHALT  PAVEMENTS.  299 

Small  repairs  of  any  breaks  that  may  occur  in  an 
asphalt  surface  may  be  easily  made,  and  such  repairs 
should  be  constantly  attended  to  in  order  to  keep  the 
surface  in  good  condition.  Small  breaks  will  rapidly 
extend  if  they  are  not  repaired  at  once.  In  making 
repairs  to  the  surface  of  the  pavement  it  is  necessary  to 
cut  away  the  surface  for  a  short  distance  about  the 
imperfect  spot,  stripping  the  surface  from  the  founda- 
tion and  cutting  the  layer  down  square  at  the  edges, 
after  which  a  new  piece  of  surface  may  be  introduced 
to  fill  the  hole  in  the  same  manner  that  the  original 
surface  was  constructed.  Such  a  patch  may  ordinarily 
be  put  on  so  as  to  make  joints  that  will  join  perfectly 
with  the  old  pavement  and  not  show  where  it  has  been 
placed.  When  a  surface  has  become  so  worn  that 
patches  would  be  numerous,  the  old  surface  may  be 
stripped  off  and  a  new  one  placed  upon  the  original 
foundation.  \Yhen  repairs  are  to  be  made  upon  a 
pavement  having  a  bituminous  base  it  is  more  difficult 
to  cut  out  the  holes  in  satisfactory  shape,  as  there  is 
no  well  defined  joint  between  the  base  and  the  surface 
layers. 

The  repairs  that  may  be  required  upon  an  asphalt 
pavement  depend,  of  course,  upon  the  solidity  of  con- 
struction and  the  nature  of  the  surface  material.  There 
is  so  great  variation  in  the  materials  employed  for  the 
wearing  surface  that,  as  would  naturally  be  expected, 
very  considerable  difference  in  wear  is  shown  by  dif- 
ferent pavements. 

It  is  common  to  require  contractors  for  asphalt 
pavements  to  guarantee  the  pavement  for  a  period  of 
years,  making  all  necessary  repairs  and  leaving  the 
work  in  good  condition  at  the  end  of  the  period.  This 
makes  it  an  object  for  the  contractor  to  do  good  work, 


300         A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

and  may  sometimes  be  the  most  effective  way  of  secur- 
ing it  where  so  many  elements  of  uncertainty  enter. 
In  general,  it  is  not  desirable  to  require  contractors  to 
guarantee  paving  for  a  long  period  on  account  of 
limiting  competition  and  increasing  unnecessarily  the 
cost  of  the  work.  With  asphalt  paving,  however, 
many  engineers  consider  the  difficulty  of  control  during 
construction,  under  ordinary  circumstances,  such  as  to 
make  a  guaranty  necessary,  while  the  fact  that  the 
material  is  for  the  most  part  controlled  by  a  few  large 
companies  renders  the  guaranty  less  undesirable  as 
restricting  competition.  This  method  has,  however, 
been  found  unsatisfactory  in  many  instances,  on  account 
of  the  difficulty  of  enforcing  the  guaranty. 

The  cost  of  maintenance  of  asphalt  pavements  varies 
widely  in  different  places,  depending  upon  the  character 
of  the  construction  used  and  the  local  conditions  sur- 
rounding the  pavement.  In  Washington,  D.  C.,  the 
average  life  of  the  surface  before  renewal  is  about  20 
years,  while  the  annual  cost  of  maintenance  is  about 
2.5  to  2.8  cents  per  square  yard  per  annum.  In  loca- 
tions where  the  surface  is  kept  continuously  damp, 
particularly  if  it  is  not  kept  clean,  the  asphalt  is  apt  to 
deteriorate  rapidly  and,  in  some  instances,  scales  off 
and  gradually  disintegrates.  The  resistance  of  asphalt 
to  water  action  depends  very  much  upon  the  density 
of  the  surface  mixture  and  the  ease  with  which  water 
may  penetrate  it.  Great  care  should  be  used  in  laying 
pavements  where  moisture  conditions  are  not  good  to 
secure  a  dense  surface  mixture  in  which  the  voids  are 
well  rilled.  Where  water  may  continuously  run  in  the 
gutters,  it  is  usually  better  to  construct  the  gutters  of 
other  material  less  affected  by  the  action  of  water. 

Injury   to    asphalt    surfaces   from   illuminating   gas 


ASPHALT  PAVEMENTS.  301 

escaping  from  leaking  mains  has  been  observed  by  Mr. 
A.  W.  Dow  at  Washington,  D.  C.  The  heavy  hydro- 
carbons of  the  gas  are  absorbed  by  the  bitumen  of 
asphalt,  which  is  thereby  softened  and  caused  to  cut 
and  flow  under  the  traffic. 

The  cost  of  maintenance  depends  largely  upon  the 
system  employed  in  the  maintenance  work.  In  some 
cities  repairs  are  made  only  at  considerable  intervals 
when  the  surface  is  in  bad  condition,  and  in  such 
instances  the  ultimate  cost  is  usually  much  larger  than 
where  small  repairs  are  made  as  they  are  needed  to  keep 
the  surface  always  in  good  condition. 

ART.  73.     BITULITHIC  PAVEMENT. 

The  name  "bitulithic"  is  commonly  applied  to  a 
pavement,  the  surface  of  which  is  composed  of  a 
bituminous  concrete,  the  aggregate  being  a  mixture  of 
several  sizes  of  broken  stone,  so  proportioned  as  to 
give  a  dense  material  with  a  small  percentage  of  voids. 
Pavements  of  bituminous  concrete  have  been  occasion- 
ally constructed  for  a  number  of  years,  but  the  intro- 
duction of  this  type  of  pavement  upon  a  considerable 
scale  began  about  1901,  when  exploited  under  a  patent 
of  the  Warren  Brothers,  and  most  of  those  since  con- 
structed have  been  under  this  patent. 

In  the  construction  of  pavements  of  this  class  the 
crushed  rock  is  screened  into  several  sizes,  which  are 
then  mixed  together  in  such  proportions  as  to  produce 
an  aggregate  with  very  small  percentage  of  voids. 
Four  to  six  screens  are  used,  varying  from  about  ij 
inches  to  TV  inch  openings.  Sufficient  quantities  of  the 
smaller  sizes  are  employed  to  fill  the  interstices  in  the 
larger  sizes;  the  relative  proportions  being  determined 


302      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

in  each  instance  by  experiment  upon  the  particular 
material  in  use.  "After  the  proportions  have  been 
determined,  the  mineral  material  is  passed  through  a 
rotary  screen  which  separates  it  into  several  different 
groups  of  sizes.  The  proper  proportion  by  weight  of 
each  of  these  sizes  is  secured  by  the  use  of  a  scale  having 
seven  beams,  the  exact  required  amount  being  weighed 
out  and  run  into  a  double  shaft  rotary  mixer.  There 
it  is  combined  with  a  bituminous  cement  which  is  also 
accurately  weighed  in  the  proper  proportion.  The 
whole  is  then  thoroughly  mixed  together  and  dumped, 
while  still  hot,  into  carts,  hauled  to  the  street,  spread, 
and  thoroughly  rolled  with  heavy  steam  road  rollers. 

"After  the  surface  is  thoroughly  rolled,  a  flush  coat 
of  quick  drying  bituminous  cement  is  applied  to  the 
surface.  There  is  then  applied  a  thin  layer  of  hot 
finely  crushed  stone,  varying  from  J  to  J  inches  in 
size,  according  to  the  roughness  of  the  surface  desired. 
The  pavement  is  again  heavily  rolled,  leaving  the  street 
in  a  finished  condition. " 

These  pavements  are  commonly  constructed  upon 
bituminous  foundations  (see  Art.  58).  When  the  sub- 
foundation  is  not  firm,  and  concrete  foundations  are 
required,  the  surface  of  the  concrete  is  roughened  by 
scattering  stone  of  about  I J  inch  diameter  lightly  over 
it,  and  ramming  the  stones  into  the  concrete  to  about 
half  their  depths.  This  forms  a  bond  between  the  base 
and  surface  of  the  pavement,  and  prevents  the  creeping 
of  the  surface. 

These  pavements  have  been  used  with  good  success 
in  many  places  throughout  the  United  States.  They 
require  care  and  skill  in  construction,  both  in  securing 
proper  grading  of  the  mineral  aggregate  and  in  the 
character  and  proportions  of  the  bituminous  cement. 


ASPHALT  PAVEMENTS.  303 

It  is  claimed  by  the  advocates  of  this  kind  of  construc- 
tion that,  on  account  of  the  density  and  firmness  of  the 
mass  of  stone  of  which  they  are  composed,  a  softer 
bitumen  may  be  employed,  thus  eliminating  the  danger 
of  cracking  in  cold  weather.  In  some  instances,  where 
hard  stone  has  been  used  in  forming  the  surfaces,  there 
are  indications  that  this  construction  will  give  better 
resistance  to  wrear  than  the  ordinary  asphalt  surface, 
but  longer  experience  is  necessary  to  fully  test  its 
durability.  It  has  been  successfully  used  upon  much 
steeper  grades  than  sheet  asphalt,  being  reported  as 
affording  a  good  foothold  to  horses,  and  satisfactory 
in  one  instance  upon  a  12  per  cent  grade. 

In  constructing  these  pavements,  as  with  sheet 
asphalt,  it  has  been  customary  to  rely  upon  the  con- 
tractor's guaranty  for  securing  good  work  and  no 
attempt  is  usually  made  to  determine  the  character  of 
the  bituminous  cement  by  direct  tests.  This  is  an 
undesirable  feature  of  most  work  with  these  materials, 
and  it  is  to  be  hoped  that,  as  better  information  con- 
cerning the  bitumens  becomes  available,  more  satis- 
factory specifications  may  become  feasible.  The  fol- 
lowing is  an  extract  from  the  specifications  used  in 
St.  Louis  in  1908: 

"Upon  the  foundation  shall  be  laid  the  wearing 
surface,  which  shall  be  composed  of  carefully  selected 
sound,  hard  crushed  stone,  mixed  with  bituminous 
cement  and  laid,  as  hereinafter  specified. 

"  The  stone  last  referred  to  shall  have  a  percentage  of 
wear  not  to  exceed  5  per  cent  when  tested  in  the  follow- 
ing manner: 

"  The  sample  to  be  tested  shall  be  broken  into  pieces 
that  will  pass,  in  all  positions,  through  a  6  centi- 
meter ring,  but  not  through  a  3  centimeter  ring.  The 


304      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

fragments  of  stone  shall  then  be  cleaned,  dried  in  a  hot 
air  bath  at  1 00°  C.  and  cooled  in  a  dessicator,  after 
which  five  kilograms  shall  be  weighed  out  and  placed 
in  a  cylinder  of  an  abrasive  machine  and  the  cover 
bolted  on.  This  machine  (see  Art.  37)  shall  consist 
of  a  cast  iron  cylinder,  or  cylinders,  fastened  to  a  shaft 
so  that  the  axis  of  each  cylinder  makes  an  angle  of 
30  degrees  with  the  axis  of  rotation.  Each  cylinder 
shall  be  20  centimeters  in  diameter  and  34  centimeters 
in  depth;  shall  be  closed  at  one  end  and  shall  have  a 
tightly  fitting  cover  at  the  other  end.  After  this, 
the  machine  shall  be  rotated  at  the  rate  of  2000  revo- 
lutions per  hour  for  five  hours.  When  the  10,000 
revolutions  of  the  machine  are  completed,  the  contents 
of  the  cylinder  shall  be  placed  on  a  sieve  of  0.16  centi- 
meter mesh,  and  the  material  which  passes  through 
carefully  collected  and  weighed.  The  ratio  between 
the  weight  of  the  fine  material  and  the  original  five 
kilograms  placed  in  the  cylinder  is  the  percentage  of 
wear. 

"  After  immersion  in  water  for  a  period  of  ninety-six 
hours,  a  smoothly  worn  fragment  of  stone  weighing 
between  20  and  60  pounds  shall  not  absorb  more  than 
3  pounds  of  water  per  cubic  foot  of  stone. 

"After  heating  the  stone  in  a  rotary  mechanical 
dryer  to  a  temperature  of  about  250°  F.  it  shall  be 
passed  through  a  rotary  screen  having  six  or  more 
.sections,  with  varying  sized  openings,  the  maxi- 
mum of  which  shall  not  be  larger  than  one  and 
one-half  inch,  and  the  minimum  one-tenth  of  an  inch 
in  diameter.  The  several  sizes  of  stone  thus  separated, 
by  the  screen  sections  shall  pass  into  a  bin  containing 
six  sections  or  compartments.  From  this  bin  the 
Gtone  shall  be  drawn  into  a  weight  box,  resting  on  a 


ASPHALT  PAVEMENTS.  3°5 

scale  having  seven  beams.  The  stone  from  each  bin 
shall  be  accurately  weighed  in  the  proportions  deter- 
mined by  laboratory  tests  that  will  give  the  greatest 
density  of  mineral  aggregate  and  the  greatest  inherent 
stability  of  the  mineral  aggregate.  From  the  weigh- 
box  each  batch  of  mineral  aggregate  composed  of 
different  sizes,  accurately  weighed,  as  above  described, 
shall  pass  into  a  "twin  pug"  or  other  appropriate 
form  of  mixer.  If  the  proportions  of  crushed  stone  in 
the  mixer  do  not  provide  enough  fine  particles  to  bring 
the  aggregate  to  the  density  desired,  there  may  be 
added  not  to  exceed  15  per  cent  of  fine  sand,  gravel, 
hydraulic  cement,  and  pulverized  limestone.  To  the 
stone  in  the  mixer  shall  then  be  added  a  sufficient 
quantity  of  Warren's  Puritan  Brand  No.  21  Bituminous 
Water-proof  Cement,  to  thoroughly  coat  all  the  particles 
of  stone  and  fill  all  the  voids  in  the  mixture.  The 
bituminous  cement  shall,  before  mixing  with  the  stone, 
be  heated  to  between  200  and  250°  F.  and  the  amount 
used  in  each  batch  shall  be  accurately  weighed  and 
used  in  such  proportion  as  have  been  previously  deter- 
mined by  laboratory  tests  to  give  the  best  results 
and  fill  the  voids  in  the  mineral  aggregate.  The 
mixing  shall  be  continued  until  the  result  is  a  uniform 
bituminous  concrete.  In  this  condition  it  shall  be 
hauled  to  the  street  and  there  spread  on  the  prepared 
foundation  to  such  depth  that  after  thorough  com- 
pression with  a  steam  roller  it  shall  have  a  thickness 
of  two  inches.  The  proportion  of  the  various  sizes  of 
stone  and  of  bituminous  cement  shall  be  such  that  the 
compressed  mixture  shall  have  as  nearly  as  possible 
the  density  of  solid  stone. 

"After  rolling  the  wearing  surface,  there  shall  be 
spread    over    it    a    thin    coating    of  Warren's    Quick 


306       A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

Drying  Bituminous  Flush  Coat  Composition  in  a  plas- 
tic condition,  for  the  purpose  of  closing  any  pores  or 
cellular  openings,  and  to  thoroughly  fill  any  uneven- 
ness  or  honeycomb  which  may  appear  in  the  surface. 
There  shall  then  be  applied  thereto  and  combined 
therewith  while  plastic,  stone  chips,  with  the  same 
qualities  required  of  the  stone  in  the  pavement 
proper,  by  rolling  the  same  into  the  surface  with 
a  heavy  steam  roller  for  the  purpose  of  presenting  a 
gritty  surface. 

"In  order  to  get  the  greatest  possible  density,  the 
pavement  shall  be  rolled  continuously  from  the  time  the 
bituminous  concrete  is  brought  upon  the  street  until 
the  stone  chips  have  been  rolled  into  the  surface  and 
the  roller  no  longer  makes  a  perceptible  impression 
upon  the  pavement. 

"  Each  layer  of  the  work  shall  be  kept  as  clean  as 
possible  so  as  to  readily  unite  with  the  succeeding 
layer.  The  bituminous  compositions  shall  in  each 
case  be  free  from  water,  petroleum  oil,  water  gas,  or 
process  tars  and  shall  be  especially  refined  with  a  view 
of  removing  the  light  oil,  naphthaline  and  other  crys- 
talline matter  susceptible  to  atmospheric  influences/* 


CHAPTER    X. 

WOOD-BLOCK  PAVEMENTS. 
ART.    74.    TYPES  OF  WOOD-BLOCK  PAVEMENT. 

THE  use  of  wood  blocks  for  the  surfaces  of  pave- 
ments began  a  little  before  1840,  and  since  that  time 
many  types  of  construction  have  been  tried  with  vary- 
ing degrees  of  success.  The  first  pavements  in  London, 
in  1839,  consisted  of  hexagonal  blocks  of  fir,  six  to 
eight  inches  in  diameter  and  about  six  inches  deep, 
placed  on  a  base  of  gravel.  In  1841  a  pavement  of 
round  beech  blocks  was  laid  upon  a  foundation  of 
planks  and  sand.  The  wood  soon  decayed  and  the 
pavement  was  removed. 

In  Philadelphia,  square  hemlock  blocks  were  laid  in 
1839  and  hexagonal  hemlock  blocks  probably  a  little 
earlier.  Both  were  quickly  destroyed  by  the  decay  of 
the  blocks.  In  New  York  and  Boston  similar  pave- 
ments were  constructed  at  about  the  same  time  and 
with  much  the  same  result. 

In  1855  a  pavement  of  tamarac  blocks  was  laid  in 
Quebec,  This  pavement  was  placed  upon  a  base 
formed  of  a  flooring  of  one  and  one-half  inch  boards 
laid  longitudinally  and  crossed  at  right  angles  by  a 
second  flooring  of  inch  boards.  A  layer  of  sand  one- 
half  inch  thick  was  placed  over  the  boards.  The 
tamarac  blocks  were  ten  to  fifteen  inches  in  diameter 
and  twelve  inches  long,  small  pieces  of  wood  being 
forced  into  the  spaces  between  the  blocks.  The 

307 


308      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

joints  were  filled  with  a  mixture  of  sand,  cement,  and 
tar.  This  heavy  construction  is  reported  as  having 
given  very  good  wear,  with  no  decay. 

Cedar  Block  Pavements.  In  the  earlier  wood  pave- 
ments of  the  United  States,  cedar  blocks  were  com- 
monly employed.  These  blocks  were  used  in  the  form 
of  whole  sections  of  the  tree  on  account  of  the  liability 
of  the  wood  to  split  off  between  the  layers  when  cut  to 
a  rectangular  shape,  as  well  as  to  reduce  waste  to  a 
minimum.  They  usually  varied  from  4  to  9  inches  in 
diameter  and  4  to  8  inches  in  depth.  In  some  cases 
the  blocks  were  cut  to  a  true  cylindrical  form,  the  sap- 
wood  as  well  as  the  bark  being  cut  away  by  passing 
the  block  through  sets  of  knives,  gauged  to  turn  out 
true  cylinders  of  given  size.  The  use  of  sapless  blocks 
increases  the  life  of  the  pavement  by  augmenting  the 
resistance  of  the  material  both  to  the  wear  of  traffic  and 
to  the  disintegrating  influences  of  the  atmosphere. 

These  pavements  were  usually  placed  upon  a  founda- 
tion of  boards  laid  upon  sand.  The  planks  were  com- 
monly tarred  and  laid  lengthwise  of  the  street,  being 
nailed  to  scantling  or  other  boards  placed  across  the 
street  and  bedded  in  the  sand.  This  construction  has 
the  disadvantage  of  lacking  firmness  as  well  as  of  being 
perishable,  although  in  some  instances  good  results  have 
been  obtained  by  its  use. 

The  construction  of  a  pavement  of  this  type  is  shown 
in  Fig.  25.  Blocks  of  varying  sizes  are  employed, 
being  set  in  contact  with  each  other  in  such  a  way  as 
to  leave  the  spaces  between  the  blocks  as  small  as 
possible.  Usually  the  joints  are  filled  with  sand  and 
gravel,  sometimes  with  a  coating  of  tar;  or  in  some 
cases  the  joint  is  partially  filled  with  tar  and  then  com- 
pletely filled  with  sand  or  small  gravel.  When  the 


WOOD-BLOCK  PAVEMENTS. 


3°9 


ordinary  coal-tar  paving  cement  filling  is  used,  the 
joints  are  first  filled  nearly  full  of  sand  or  gravel,  which 
is  pounded  down  with  a  bar,  after  which  the  hot  cement 
is  poured  in  until  the  joint  is  well  filled. 

These  pavements,  on  account  of  the  plentiful  supply 
of  timber  were  constructed  for  very  low  first  cost  and 
undoubtedly  served  a  very  useful  purpose  in  many 
instances,  permitting  the  improvement  of  many  streets 
to  an  extent  which  at  that  time  \vould  have  otherwise 
been  impossible.  They  lifted  the  streets  out  of  the 
mud,  although  the  pavements  did  not  usually  last  long 


FIG.  25. 

and  were  afterward  replaced  by  more  durable  materials. 
Pavements  of  this  type  wear  rapidly  under  traffic,  soon 
becoming  uneven,  and  their  use  has,  for  the  most  part, 
been  discontinued  on  account  of  their  lack  of  economy. 
Nicholson  Pavement.  Rectangular  wooden  blocks 
set  like  the  cedar  blocks,  upon  a  plank  foundation  were 
at  one  time  quite  extensively  used  and  known  as 
Nicholson  pavements.  In  these  pavements  the  blocks 
are  set  with  their  longest  dimension  transverse  to  the 
length  of  the  street.  They  are  usually  arranged  in 
courses  across  the  street,  being  placed  close  together 
in  the  courses,  and  arranged  to  break  joints  in  adjoining 
courses.  Between  courses  a  ioint  is  usually  made  J  to 


310      A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

J  inch  in  width  for  the  purpose  of  affording  a  foothold 
to  horses.  In  the  older  pavements  of  this  character  a 
much  wider  joint  was  employed,  some  as  much  as  an 
inch  in  width,  with  the  idea  that  they  were  necessary 
to  secure  proper  foothold.  The  joints  were  rilled  in  the 
same  manner  as  in  the  round  block  pavement.  These 
pavements  like  the  cedar  blocks  have  given  place  for 
the  most  part  to  more  economical  kinds  of  construction. 
Rectangular  Blocks  on  Concrete.  The  use  of  a  concrete 
base  under  a  surface  of  the  Nicholson  type  effected  a 
marked  improvement  in  the  wear  of  the  pavement. 


FIG.  26. 

Round  block  pavements  were  also  sometimes  placed 
upon  a  concrete  foundation. 

In  using  a  concrete  foundation  a  cushion  coat  of 
sand  is  commonly  employed  on  top  of  the  concrete  in 
which  to  bed  the  blocks  in  order  that  they  may  be 
brought  to  an  even  surface.  Sometimes  a  thin  layer 
of  cement  mortar  is  used  in  place  of  the  sand  upon  the 
concrete;  and  in  London  some  pavements  have  been 
constructed  with  a  thin  layer,  about  \  inch,  of  asphalt 
mastic  over  the  concrete,  the  blocks  resting  upon  th" 
mastic. 

A  pavement  of  this  type  is  shown  in  Fig.  26. 

In  laying  a  pavement  of  this  kind  a  course  of  blocks 


WOOD-BLOCK  PAVEMENTS.  311 

is  first  set  across  the  street,  and  then  a  strip  of  wood 
of  the  thickness  of  the  joint  is  set  against  the  row  of 
blocks  and  left  until  the  next  course  is  placed,  or 
sometimes  spuds  with  heads  of  the  thickness  of  the 
joints  are  driven  to  the  head  in  the  side  of  each  block, 
and  the  next  row  of  blocks  are  set  against  the  spuds. 

In  some  pavements  of  this  kind  hydraulic  cement 
is  employed  in  filling  the  joints,  and  in  some  instances 
the  lower  half  of  the  joint  is  filled  with  coal  tar  paving 
cement  and  the  upper  half  with  rrydraulic  cement 
mortar.  The  cement  mortar  gives  a  harder  wearing 
surface,  and  protects  the  pitch  from  the  softening  action 
of  the  sun  in  warm  weather.  In  later  practice  the 
width  of  joint  has  been  graduall\T  reduced  until  the 
blocks  are  set  in  contact  with  each  other,  occasional 
expansion  joints  being  provided. 

These  pavements  have  been  extensively  used  in 
England,  and  to  a  smaller  extent  in  the  United  States. 
They  have  been  fairly  satisfactory  in  use  but  have 
been,  for  the  most  part,  superseded  by  treated  blocks. 

Treated  Block  Pavement.  Wood  blocks  treated  by 
some  preservative  process  for  the  purpose  of  preventing 
decay  and  of  hardening  the  block  so  as  to  give  better 
resistance  to  wear  have  come  into  use  somewhat 
extensively  since  about  1900.  These  pavements  have 
given  good  service  in  use,  under  heavy  traffic  in  the 
business  sections  of  the  large  cities,  and  on  important 
residence  streets,  where  the  somewhat  high  cost  is  not 
prohibitive.  The  use  of  untreated  blocks  has  been  prac- 
tically abandoned  as  uneconomical. 

ART.  75.    WOOD  BLOCKS. 

Wood-block  pavements  are  constructed  of  blocks  set 
with  the  fibers  vertical,  so  that  wear  comes  upon  the 


312      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ends  of  the  fibers  and  has  no  tendency  to  split  pieces 
off  from  the  blocks.  These  blocks  are  usually  from  6 
to  12  inches  in  length,  3  to  4  inches  in  width,  and  3  to  4 
inches  in  depth.  Some  engineers  require  the  blocks  to 
be  of  uniform  length  but  a  variation  of  from  about  6  to 
10  inches  is  more  common  and  seems  desirable  because 
of  the  greater  freedom  in  obtaining  timber  for  the  pur- 
pose. A  depth  of  3J  inches,  or  at  most  4  inches,  is 
sufficient  and  there  seems  to  be  no  advantage  in  greater 
depth,  as  the  block  would  become  unserviceable  and 
need  to  be  renewed  before  this  depth  would  be  worn 
away.  The  blocks  are  cut  from  planks  of  uniform 
thickness,  and  are  set  in  courses  across  the  street,  the 
blocks  in  adjoining  courses  breaking  joints  with  each 
other. 

Kinds  of  Wood.  Wood  for  pavements  should  be 
close-grained  and  not  too  hard.  It  should  be  as  homo- 
geneous as  possible  in  order  that  the  wear  may  be 
uniform,  and  soft  enough  that  it  may  not  wear  smooth 
and  slippery.  The  blocks  should  always  be  subjected 
to  careful  inspection,  and  only  sound  and  well-seasoned 
timber  should  be  employed.  Blocks  containing  shakes 
and  knots  should  be  rejected,  and  when  untreated  blocks 
are  to  be  used,  all  sapwood  needs  to  be  removed  in  order 
to  lessen  the  liability  to  early  decay. 

In  Australia  hard-wood  blocks  have  been  quite 
extensively  used  and  are  reported  as  giving  good  ser- 
vice, although  they  are  admitted  to  be  somewhat  slippery 
in  wet  weather.  Australian  Karri  and  Jarrah  woods 
are  employed,  and  it  is  claimed  for  them  that  they  show 
unusually  great  resistance  to  wear  and  are  not  soon 
affected  by  decay.  These  woods  are  too  dense  for  pre- 
servative treatment  and  are  used  in  the  form  of  untreated 
blocks. 


WOOD-BLOCK  PAVEMENTS.  313 

In  London,  where  wood  pavements  have  been  very 
extensively  employed,  Swedish  yellow  deal  is  commonly 
placed  at  'the  head  of  the  list  of  woods  in  value,  yellow 
pine  and  Baltic  fir  being  also  largely  used  and  considered 
good  in  use.  The  Australian  woods  above  mentioned 
have  also  been  used  to  some  extent  in  London,  and  are 
said  to  have  given  very  satisfactory  service,  show- 
ing greater  resistance  to  wear  than  deal  or  pine,  although 
somewhat  expensive.  Deal  treated  with  creosote  is 
extensively  used  and  seems  to  give  the  best  satisfaction. 
In  Paris,  teak,  karri,  and  pitch  pines  are  frequently 
employed,  although  treated  native  pines  are  more 
commonly  used  and  have  been  found  to  give  good 
service. 

In  the  United  States,  Southern  yellow  pine  has  been 
most  extensively  employed,  and  seems  to  have  given  the 
best  satisfaction  in  use,  but  is  so  much  in  demand  for 
other  purposes  that  properly  selected  timber  is  rather 
expensive.  Norway  pine,  tamarack  and  Southern  black 
gum  are  also  used  to  some  extent  and  have  given  good 
service.  White  birch  and  hemlock  have  also  been 
successfully  tried  for  this  purpose,  although  they  have 
only  been  used  in  experimental  pavements.  All  of 
these  woods  are  used  as  treated  blocks,  and  are  readily 
susceptible  to  impregnation  with  the  creosote  oils  used 
in  treatment. 

The  specifications  used  in  New  York  City  in  1911 
require  the  blocks  to  be  of  Southern  long-leaf  yellow  pine 
or  of  Southern  black  gum,  while  the  St.  Louis  specifica- 
tions permit  the  use  only  of  the  Southern  long-leaf  yellow 
pine  blocks,  which  must  meet  the  following  requirements : 

"  Each  block  shall  consist  of  at  least  seventy-five 
(75)  per  cent  heart,  and  shall  be  free  from  bark,  large, 
loose  or  rotten  knots,  and  shakes  or  other  defects.  No 


314      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

dead  or  second-growth  timber  will  be  accepted,  and  all 
timber  shall  be  cut  in  a  locality  recognized  as  producing 
the  timber  described. 

"  The  number  of  annual  rings  shall  average  not  less 
than  six  (6)  per  inch,  measured  radially.  All  timber 
for  the  blocks  shall  be  thoroughly  air-seasoned." 

It  is  customary  to  require  that  the  timber  shall  be 
subject  to  inspection  at  the  works,  before  being  cut 
into  blocks,  or  treated  with  oil. 

The  specifications  adopted  by  the  Association  for 
Standardizing  Paving  Specifications  in  1911,  contain 
the  following  requirements: 

"  The  wood  to  be  treated  shall  consist  of  Southern 
yellow  pine,  Norway  pine,  black  gum  and  tamarack, 
only  one  kind  of  wood,  however,  to  be  used  on  any  one 
contract. 

"  Yellow  pine  blocks  shall  be  made  from  what  is 
known  as  Southern  yellow  pine,  well  manufactured, 
full  size,  saw-butted,  all  square  edged,  and  shall  be  free 
from  the  following  defects:  unsound,  loose  and  hollow 
knots,  worm  holes  and  knot  holes,  through  shakes  and 
round  shakes  that  show  on  the  surface.  In  yellow  pine 
timber  the  annual  rings  shall  average  not  less  than  eight 
to  the  inch,  and  shall  in  no  case  be  less  than  four  to  the 
inch  measured  radially. 

"  Norway  pine  block,  gum  and  tamarack  block  shall 
be  cut  from  timber  that  is  first  class  in  every  respect,  and 
shall  be  of  the  same  grade  as  that  defined  for  the  Southern 
yellow  pine. 

"  The  blocks  shall  be  from  5  to  10  inches  long,  but 
shall  average  8  inches;  they  shall  be  three  and  one-half 
(3^)  and  four  (4)  inches  in  depth,  according  to  traffic; 
they  shall  be  from  three  (3)  to  four  (4)  inches  in  width; 
but  all  blocks  in  one  street  or  improvement  shall  be  of 


WOOD-BLOCK  PAVEMENTS.  315 

uniform  width,  provided  that  blocks  3  inches  in  depth 
can  be  used  on  residential  streets  and  in  alleys;  pro- 
vided, further,  that  in  no  case  shall  the  width  and  depth 
of  blocks  be  equal. 

"  A  variation  of  one  sixteenth  {re)  °^  an  ^ncn  shall  De 
allowed  in  the  depth  and  one  eighth  (J)  of  an  inch  in  the 
width  of  the  blocks." 


ART.  76.    TREATMENT  OF  WOOD  BLOCKS. 

The  most  serious  objection  commonly  raised  to  the 
older  type  of  wood  pavement  is  that  wood,  being  porous, 
absorbs  moisture  readily,  and  is  thus  liable  both  to 
destruction  through  decay  and  to  become  injurious  to 
health.  Various  methods  were  therefore  proposed  for 
rendering  the  blocks  less  pervious  and  more  durable 
by  impregnating  them  with  various  substances  which 
fill  the  pores  and  act  as  preservatives.  The  earlier 
attempts  in  this  direction  were  not  in  the  main  success- 
ful and  little  seemed  to  be  gained  in  durability  by  the 
treatment.  Solutions  of  mineral  salts  were  tried  but 
were  found  unsuitable  for  the  purpose.  Creosoting  the 
blocks,  which  consists  in  impregnating  the  wood  with 
the  oil  of  tar,  or  creosote,  was  more  successful,  but 
with  the  type  of  construction  in  use  seemed  of  doubt- 
ful economic  value. 

In  the  process  of  creosoting,  the  wood  is  first  thor- 
oughly dried,  usually  by  heating  it  in  a  kiln,  and  the 
hot  creosote  is  then  forced  in  under  pressure.  The 
method  of  accomplishing  this  varies  in  different  places. 
In  order  to  be  effective  the  process  must  be  thoroughly 
carried  out  and  the  pores  well  filled.  It  has  been  com- 
monly recommended  that  from  8  to  12  pounds  of  creosote 


316      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

per  cubic  foot  of  timber  should  be  forced  in,  as  a  min- 
imum requirement  for  the  softer  woods,  such  as  are 
commonly  used  in  pavements.  Creosote  has  the  prop- 
erty of  destroying  the  lower  forms  of  animal  life,  and  is 
therefore  an  effective  preservative  against  destruction 
through  these  agencies  where  they  exist.  It  is  therefore 
often  employed  for  the  preservation  of  timber  for  sub- 
aqueous construction  in  sea-water.  This  process,  when 
properly  applied,  is  effective  in  preventing  decay,  and 
therefore  in  lengthening  the  natural  life  of  the  wood. 
It  also  renders  the  wood  less  permeable  and  thus  removes 
the  objection  to  the  old  form  of  pavement  based  upon 
its  absobent  nature. 

The  earlier  efforts  to  increase  the  life  of  wood  pavements 
by  the  use  of  creosoted  blocks  were  rather  unsuccessful. 
The  resistance  of  the  wood  to  the  wear  of  the  traffic  did 
not  seem  to  be  materially  affected  by  the  treatment  given, 
and  the  failure  of  the  pavement  being  ordinarily  from 
wear  rather  than  from  decay,  the  life  of  the  pavement 
was  not  materially  prolonged.  More  recently,  however, 
careful  methods  of  treatment  have  produced  blocks 
which  show  high  resistance  to  wear,  and  give  good  results 
under  the  heavy  traffic  of  trie  large  cities.  The  present 
treatment  is  more  thorough,  a  larger  quantity  of  oil 
being  injected,  and  in  most  instances,  heavier  oils  being 
employed.  The  object  is  to  waterproof  the  blocks  by 
completely  rilling  the  pores  of  the  wood.  It  is  also 
claimed  that  heavy  oils  harden  in  the  pores  and  add  to 
the  resistance  to  wear  by  preventing  displacement  of  the 
wood  fiber,  when  subjected  to  loads  or  blows. 

The  heavy  oil  of  tar,  known  as  creosote  oil  in  the 
preservation  of  timber  for  other  purposes,  is  commonly 
used,  but  is  frequently  combined  with  other  materials 
intended  to  give  greater  stability  and  more  thorough 


WOOD-BLOCK  PAVEMENTS.  317 

waterproofing.  There  is  .much  difference  of  opinion 
concerning  the  efficiency  of  the  various  oils  employed. 

Creo-resinate  Process.  Many  of  the  pavements  now 
in  use  have  been  constructed  by  the  creo-resinate  process. 
In  the  creo-resinate  treatment,  the  wood  is  impregnated 
with  a  substance  consisting  essentially  of  a  mixture  of 
the  oil  of  tar  with  resin,  the  resin  acting  as  the  water- 
proofing and  hardening  material.  The  amount  of  resin 
required  varies  from  about  25  per  cent  to  50  per  cent 
of  the  mixture,  and  depends  upon  the  character  of  the 
oil  used,  a  heavy  dense  oil  requiring  less  resin  than  a 
lighter  and  more  volatile  oil.  The  method  of  treat- 
ment is  thus  described  by  Mr.  F.  A.  Kummer:* 

"  Blocks,  after  being  cut  to  size,  are  placed  in  circular 
cages  made  of  band  steel  of  approximately  the  diameter 
of  the  cylinders  in  which  the  treatment  takes  place, 
and  are  then,  while  in  these  cages,  run  into  the  cylin- 
ders on  cars.  The  cylinders  themselves  are  usually 
about  6  feet  in  diameter  and  somewhat  over  TOO  feet 
long,  and  are  provided  with  steam  coils  along  the  bot- 
tom and  sides  to  provide  heat  for  drying  and  preparing 
the  lumber  for  treatment.  The  blocks  are  heated  in 
this  way,  some  works  employing  live  steam  instead  of 
steam  coils,  and  others  a  combination  of  the  two.  After 
several  hours  both  by  the  use  of  heat  and  by  the  use  of  a 
vacuum  pump,  a  large  portion  of  the  moisture  and  light 
volatile  oils  in  the  wood,  if  the  latter  contain  any  such, 
are  driven  off.  The  preservative  material  is  run  into 
the  cylinder  under  a  vacuum  and  hydraulic  pressure 
of  200  pounds  per  square  inch,  applied  from  two  to  three 
hours,  or  for  such  longer  period  of  time  as  may  be  nec- 
essary to  thoroughly  treat  the  charge,  the  result  being 

*  Engineering  Record,  August  25,  1906. 


A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

accomplished  when  the  gauges  show  that  no  more  material 
is  entering  the  wood." 

This  method  was  specified  for  a  number  of  years  in 
the  City  of  New  York,  but  is  no  longer  required  on 
account  of  the  high  cost. 

Creosote  Oil.  Sometimes  so-called  creosote  oil  is  used 
without  the  admixture  of  heavier  materials.  The  advocates^ 
of  this  method  claim  that  a  more  thorough  impregna- 
tion of  the  wood  is  possible  than  with  the  heavier  materials, 
that  the  results  are  equally  good  in  durability,  and  that 
the  material  h  more  easily  obtainable.  The  character 
of  the  oil  used  is  shown  by  the  following  extract  from 
specifications  recommended  by  the  Wyckoff  Pipe  and 
Creosoting  Company.  These  specifications  require  the 
injection  of  at  least  18  pounds  of  the  oil  into  yellow  pine 
blocks : 

"  The  creosote  oil  shall  be  a  dead  oil  of  coal  tar  or 
coal-tar  product.  It  shall  not  contain  more  than  3  per 
cent  of  water  and  if  it  does  contain  this  amount  of  water 
a  corresponding  correction  must  be  made  so  that  an 
equivalent  additional  amount  of  creosote  is  forced  into 
the  blocks.  It  shall  contain  only  traces  of  acetic  acid  and 
acetates.  Its  specific  gravity  at  100°  F.  (38°  C.)  shall 
be  at  least  1.03  and  not  more  than  1.07  so  as  to  assure 
its  thoroughly  penetrating  the  wood  blocks.  The  residue 
insoluble  by  filtration  with  benzol  and  chloroform  must 
not  exceed  3  per  cent  of  the  weight  of  the  creosote  oil. 
Fractional  distillation  of  100  grams  of  the  creosote  oil 
shall  produce  percentages  of  dry  oil  by  weight  within  the 
following  limits : 

Up  to  150°  C.  (302°  F.)  not  to  exceed      2% 

Between  150°  C.  (302°  F.)  and  170°  C.  (338°  F.)  not  to  exceed      1.5% 

Between  170°  C.  (338°  F.)  and  235°  C.  (455°  F.)  not  to  exceed  35% 

Between  235°  C.  (455°  F.)  ar.d  300°  C.  (572°  F.)  not  to  exceed  35% 


WOOD-BLOCK   PAVEMENTS.  319 

The  residue  remaining  shall  be  soft  and  adhesive. 
The  creosote  oil  shall  contain  about  25  per  cent  of 
crystallizable  naphthalene  and  at  least  15  per  cent  anthra- 
cene oils.  At  least  95  per  cent  of  the  creosote  oil  shall 
be  soluble  in  carbon  bisulphide  and  equally  in  absolute 
alcohol." 

Heavy  Tar  Oils.  The  use  of  tar  oils  of  greater  specific 
gravity  than  the  ordinary  creosote  oils,  has  been  rapidly 
developing  in  the  past  few  years.  These  c.ls  are  pro- 
duced by  adding  coal-tar  pitch  to  creosote  oil  so  as  to 
obtain  an  oil  of  proper  consistency.  It  is  claimed  by 
advocates  of  these  oils  that  they  more  completely  water- 
proof the  block,  and  make  it  more  resistant  to  wear. 
The  Association  for  Standardizing  Paving  Specifications 
has  recommended  this  method  of  treatment.  These 
specifications  have  been  criticised  on  the  ground  that  the 
oil  is  difficult  to  obtain  and  is  a  monopoly.  This,  how- 
ever, the  association  does  not  think  to  be  the  case.  The 
objection  is  also  made  that  these  heavy  tar  oils  make  a 
pavement  which  becomes  sticky  and  disagreeable  in 
hot  weather.  The  association,  however,  are  of  opinion 
hat  this  is  the  most  desirable  method  of  treatment,  and 
in  1911  adopted  the  following  specifications: 

Preservative.  The  preservative  to  be  used  shall  be  a 
coal-tar  product,  free  from  adulteration  of  any  kind 
whatever,  and  shall  comply  with  the  following  require- 
ments : 

1.  The  specific  gravity  shall  not  be  less  than  i.io  or 
more   than    1.14   at   a    temperature   of   thirty-eight    (38) 
degrees  Centigrade. 

2.  Not  more  than  three  and  one-half  (3^)  per  cent  of 
the  oil  shall  be  insoluble  by  hot  continuous  extractions 
with  benzol  and  chloroform. 

3.  On   distillation,   which   shall   be   made   exactly   as 


320      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

described  in  Bulletin  No.  65  of  the  American  Railway 
Engineering  and  Maintenance  of  Way  Association,  as 
shown  in  the  appendix  to  these  specifications,  the  dis- 
tillate shall  not  exceed  two  (2)  per  cent  up  to  150  degrees 
Centigrade,  and  shall  not  be  less  than  thirty  (30)  or 
more  than  forty  (40)  per  cent  up  to  315  degrees  Centigrade. 

4.  The  manufacturer  of  the  oil  shall  permit  full  and 
complete  inspection  and  sampling  at  the  factory  at  which 
the  oil  is  produced,  of  all  materials  either  crude  or  refined, 
entering  into  the  manufacture  of  the  finished  product, 
as  well  as  the  finished  product  itself,  in  order  that  the 
materials  used  can  be  determined  to  be  in  accordance 
with  the  foregoing  requirements.  He  shall  also  submit 
satisfactory  proof  of  the  origin  of  all  materials  entering 
into  the  composition  of  the  finished  product. 

Samples  of  the  preservative  taken  by  the  inspector  from 
the  treating  tank  during  the  progress  of  the  work  shall 
at  no  time  be  allowed  to  show  an  accumulation  of  more 
than  two  (2)  per  cent  of  foreign  matter,  such  as  sawdust 
and  dirt. 

Due  allowance  shall  be  made  for  such  accumulation 
of  foreign  matter  by  injecting  an  additional  quantity 
of  oil  into  the  blocks. 

Treatment.  The  blocks  shall  be  treated  with  the 
preservative  elsewhere  described,  so  that  the  pine  and 
tamarack  blocks  shall  contain  not  less  than  twenty  (20) 
pounds,  and  the  gum  blocks  not  less  than  twenty-two 
(22)  pounds  per  cubic  foot. 

This  amount  may  be  reduced  to  sixteen  (16)  pounds, 
under  conditions  of  heavy  traffic  in  the  discretion  of  the 
engineer. 

Inspection.  The  party  manufacturing  the  blocks  shall 
equip  his  plant  with  all  necessary  gauges,  appliances 
and  facilities  to  enable  the  inspector  to  satisfy  himself 


WOOD-BLOCK  PAVEMENTS.  321 

that    the    requirements    of    the    specifications    are    ful- 
filled. 

ART.  77.    TESTS  FOR  WOOD  BLOCKS. 

Specification  requirements  for  wood  blocks  vary 
widely  in  different  places  throughout  the  country  and 
no  systematic  method  has  been  adopted  for  the  inspec- 
tion and  testing  of  the  blocks.  A  number  of  tests  have 
been  proposed  for  use  both  at  the  plant  where  the  blocks 
are  treated  and  after  the  blocks  have  been  delivered  at 
the  site  of  the  pavement.  Inspection  of  the  blocks 
is  frequently  made  before  treatment,  as  well  as  tests 
of  the  oil  to  be  used  in  the  treatment,  an  inspector  being 
kept  at  the  plant  for  the  purpose  while  the  blocks  are 
being  prepared.  The  thoroughness  of  the  treatment 
is  determined  by  the  difference  in  weight  of  treated 
and  untreated  blocks. 

For  the  examination  of  wood  blocks  after  their  delivery 
at  the  point  of  use,  several  tests  are  in  use  or  have  been 
proposed : 

a.  The  blocks  are  inspected  as  to  their  size,  shape, 
a  ad  freedom  from  defects. 

b.  Blocks  may  be  split  and  examined  as  to  the  thor- 
oughness of  the  treatment,   and  a  weight  test  applied 
to  determine  whether  a  sufficient  quantity  of  oil  has  been 
absorbed  by  the  block. 

6.  Tests  of  absorption  are  made  by  first  drying  the 
blocks  and  then  soaking  them  in  water,  thus  determining 
the  amount  of  water  that  may  be  absorbed. 

The  1911  specifications  of  the  City  of  New  York  con- 
tain the  following  requirement: 

"  After  treatment  the  blocks  are  to  show  such  water- 
proof qualities  that  after  being  dried  in  an  oven  at  a 
temperature  of  100°  F.  for  a  period  of  24  hours,  weighed 


322      A   TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

and  then  immersed  in  clear  water  for  a  period  of  24  hours 
and  again  weighed,  the  gain  in  weight  is  not  to  be  more 
than  3^  per  cent  for  pine  blocks  and  4  4  per  cent  for  gum 
blocks." 

This  test  is  made  upon  blocks  as  delivered  for  use  in 
order  that  the  effect  of  drying  out  after  treatment  may  be 
included  in  the  results. 

d.  The  character  of  the  oil  with  which  the  block  has 
been  treated  is  tested  by  extracting  the  oil  with  carbon 
bisulphide  and  then  subjecting  it  to  tests  to  determine 
whether  it  conforms  to  the  specifications  for  oil  to  be 
used  in  the  treatment.     Fine  turnings  from  the  block  are 
placed  in  an  extraction  apparatus  with  the  solvent,  and 
the   oil   completely    extracted.     The    separation   of   the 
creosote  oil  from  the  solution  is  effected  by  distillation, 
the  solvent  being  first  removed  at  a  temperature  of  about 
120°  C.,  and  the  creosote  oil  below  about  370°  C.     The 
creosote  oil   thus   separated   is  then   subjected   to  tests 
to  determine  whether  it  meets  the  specification  require- 
ments. 

e.  It   has   been   proposed    to    test    the    resistance    to 
abrasion  of  the  blocks  by  grinding  them  upon  a  disk 
machine,  but  no  records  are  available  as  to  results  obtained 
in  such  tests,  and  they  are  of  doubtful  utility. 

A  standard  method  of  making  analyses  of  creosote 
has  been  adopted  by  the  American  Railway  Maintenance 
of  Way  Association,  and  is  now  commonly  employed  in 
making  such  examinations. 

ART.  78.    CONSTRUCTION  OF  WOOD  PAVEMENTS. 

As  stated  in  Art.  74  the  older  types  of  wood-block 
pavement,  in  which  the  blocks  were  laid  with  open 
joints  on  a  plank  foundation  or  on  gravel,  are  prac- 


WOOD-BLOCK  PAVEMENTS.  323 

tically  obsolete,  and  wood  blocks  are  usually  laid  with 
close  joints  upon  concrete  foundations.  This  gives 
firm  support  to  the  blocks  and  admits  of  even  wear 
upon  the  surface  of  the  pavement.  A  durable  base 
also  has  the  advantage  that  when  the  surface  layer  is 
worn  out,  the  pavement  may  be  resurfaced  without 
removing  the  foundation.  The  concrete  base  is  con- 
structed in  the  ordinary  manner  as  described  in  Art.  57. 
It  is  commonly  about  6  inches  thick,  although  under 
specially  trying  conditions  a  somewhat  greater  thick- 
ness is  sometimes  employed.  In  a  few  of  the  European 
pavements  very  heavy  foundations,  7  or  8  inches  thick, 
are  employed;  but  these  are  exceptional  and  the  6  inch 
depth  is  usually  found  sufficient.  Lighter  foundations, 
4  or  5  inches  in  depth  may  be  used  under  favorable 
conditions  and  where  traffic  is  not  heavy;  but  these 
pavements  are  usually  employed  upon  streets  of  con- 
siderable traffic,  and  in  such  situations  very  light 
construction  is  not  desirable. 

For  the  purpose  of  receiving  the  blocks  and  affording 
them  uniform  support,  a  cushion  coat  of  sand  or  a 
thin  coating  of  cement  mortar  is  placed  over  the  con- 
crete. The  sand  cushion  when  used  is  usually  about 
I  inch  in  thickness  and  is  placed  in  the  same  manner 
as  in  laying  a  brick  pavement.  When  a  mortar  surface 
is  employed,  a  coating  of  about  £  inch  of  mortar  is 
floated  over  the  surface  of  the  concrete  and  brought  to 
the  exact  form  of  the  finished  surface,  the  blocks  being 
placed  before  the  mortar  sets  and  bedded  into  the 
surface  of  the  mortar.  This  method,  while  used  to  a 
much  less  extent  than  the  sand  cushion,  seems  to  give 
excellent  results  in  maintaining  a  uniform  surface 
where  the  work  is  properly  done,  giving  more  uniform 
support  than  the  sand  cushion. 


324      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

The  blocks  are  set  with  the  grain  vertical,  close 
together  and  commonly  in  courses  making  an  angle 
of  60  to  70  degrees  with  the  curb  line.  In  some 
instances  the  blocks  are  placed  with  open  joints  across 
the  street  of  J  to  J  inch.  Most  of  the  older  work  was 
constructed  in  this  manner,  the  wide  joints  being 
intended  to  give  better  foothold  to  horses,  as  well  as  to 
allow  for  expansion.  Expansion  is  commonly  provided 
for  by  an  expansion  joint  along  the  curb,  although  in 
some  instances  such  joint  is  not  used.  These  joints  are 
filled  with  bituminous  cement,  and  are  usually  about 
f  of  an  inch  wide  for  streets  not  more  than  50  feet  in 
width.  Sometimes  expansion  joints  are  used  across  the 
street  at  distances  of  about  100  feet  apart,  although  the 
use  of  such  joints  is  diminishing,  and  under  ordinary 
circumstances  do  not  seem  to  be  necessary. 

After  the  blocks  are  set,  they  are  rolled  with  a  small 
steam  roller  until  the  surface  is  smooth  and  even.  When 
the  blocks  are  placed  on  a  mortar  bed,  the  rolling  must 
be  completed  before  the  mortar  has  set. 

Filling  the  Joints.  Three  methods  of  filling  joints 
are  frequently  employed,  sand,  pitch  and  cement  mortar 
being  used.  Sand  filler  is  applied  by  placing  a  light 
coating  of  dry  sand  over  the  surface  and  brushing  it 
into  the  joints,  then  covering  the  pavement  with  a  layer 
of  sand  and  opening  the  street  to  traffic.  Pitch  joints 
are  made  by  the  use  of  asphalt  or  coal-tar  paving  cement 
as  used  for  brick  pavements.  They  are  made  by  spreading 
the  hot  paving  cement  over  the  surface  and  brushing 
into  the  joints.  Care  must  be  used  to  apply  the  cement 
at  such  temperature  as  will  cause  it  to  readily  run  into 
the  joints  and  brush  off  all  surplus  cement.  A  light 
coating  of  sand  is  then  placed  over  the  pavement  to 
take  up  and  grind  off  the  pitch  left  on  the  surface  of 


WOOD-BLOCK  PAVEMENTS.  325 

the  pavement,  which  may  otherwise  become  objection- 
able in  warm  weather. 

When  the  joints  are  grouted  with  hydraulic  cement, 
a  mortar  composed  of  one  part  cement  to  two  parts 
sand  is  usually  employed,  mixed  to  a  liquid  condition 
so  that  it  may  easily  run  into  the  joints.  This  mortar 
is  slushed  upon  the  surface  and  broomed  into  the 
joints,  and  a  light  coating  of  sand  is  placed  over  the 
surface  before  opening  to  traffic.  This  sand  is  ground 
by  the  traffic  into  the  blocks,  tending  to  make  the  sur- 
face more  gritty. 

Following  are  extracts  from  specifications  recommended 
by  a  Committee  of  the  American  Society  of  Municipal 
Improvements  in  1910: 

"Laying  Blocks.  Upon  the  bed  thus  prepared  the 
blocks  shall  be  carefully  set  with  the  fiber  of  the  wood 
vertical  in  straight  parallel  courses  at  right  angles  to  the 
curb,  except  that  one  row  of  block  shall  be  placed  parallel 
with  the  curb  and  f  of  an  inch  therefrom.  The  space 
thus  formed  between  the  curb  and  this  row  of  blocks 
shall  be  filled  with  a  bituminous  filler  having  a  penetra- 
tion between  30  and  40  when  tested  at  77°  F.  On  streets 
50  feet  or  more  in  width,  a  second  row  of  blocks  parallel 
to  the  first  row  along  the  curb  and  J  of  an  inch  therefrom, 
shall  be  laid  and  the  space  between  the  two  rows  filled 
with  a  bituminous  filler  as  above,  thus  forming  a  double 
expansion  joint. 

"  When  deemed  advisable  by  the  engineer  on  streets 
for  heavy  traffic  the  row  or  rows  of  blocks  parallel  with 
the  curb  and  the  expansion  joint  may  be  dispensed  with. 

''  The  blocks  should  be  laid  by  setting  them  loosely 
together  on  the  cushion  coat,  but  no  joint  shall  be  more 
than  J  of  an  inch  in  width.  Nothing  but  whole  blocks 
shall  be  used,  except  in  starting  a  course  or  in  such  other 


326      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

cases  as  the  city  may  direct,  and  in  no  case  shall  less 
than  one-third  of  a  block  be  used  in  breaking  joints. 
Closures  shall  be  carefully  cut  and  trimmed  by  experienced 
men.  The  portions  of  the  blocks  used  for  closure  must 
be  free  from  check  or  other  fracture,  and  the  cut  end 
must  have  surface  perpendicular  to  the  top  of  the  block 
and  cut  to  the  proper  angle  to  give  a  close,  tight  joint. 

''After  the  blocks  are  placed,  they  shall  be  rolled  by  a 
small  steam  roller  until  the  surface  becomes  smooth 
and  is  brought  truly  to  the  grade  and  contour  of  the 
finished  pavement.  When  laid  on  a  mortar  bed,  the 
rolling  shall  be  completed  before  the  mortar  has  set, 
and  all  mortar  that  has  set  before  the  blocks  are  in  place 
and  rolled,  shall  be  discarded  and  replaced  by  fresh 
mortar. 

" After  the  blocks  have  been  thoroughly  rolled,  the 
joints  between  them  shall  be  filled  by  sweeping  with  fine, 
clean,  dry  sand,  all  of  which  shall  pass  a  ten-mesh  sieve. 

"  After  inspection  by  the  proper  city  official,  the  surface 
of  the  wood  block  pavement  shall  be  covered  to  a  depth 
of  about  -2  inch  with  fine  screened  sand.  This  sand  is 
to  be  left  upon  the  pavement  for  such  time  as  may  be 
directed  by  the  proper  city  official,  after  which  it  shall 
be  swept  up  and  taken  away  by  the  contractor." 

ART.  79.    MAINTENANCE  OF  WOOD  PAVEMENTS. 

The  ordinary  maintenance  of  wood  pavements,  like 
that  of  most  other  pavements,  consists  in  keeping  the 
pavement  clean  and  in  repairing  from  time  to  time  any 
small  breaks  that  may  appear  in  the  surface  due  to  im- 
perfect material  or  to  the  settling  of  the  foundation. 
These  repairs  would,  of  course,  include  the  removal  of 
any  defective  blocks  and  the  taking  up  and  replacing  of 


WOOD-BLOCK  PAVEMENTS.  327 

any  portion  which  may  settle  out  of  surface  through 
inefficient  support. 

It  is  generally  agreed  that  the  wear  of  a  wood  surface 
is  improved  by  giving  it  an  occasional  coating  of  small 
gravel,  in  some  cases  two  or  three  times  a  year,  and 
permitting  it  to  be  ground  into  the  surface  for  a  few 
days. 

When  the  wood  pavement  needs  renewal  or  exten- 
sive repairs  the  surface  may  be  relaid  as  with  any  other 
block  pavement:  if  a  permanent  foundation  be  em- 
ployed, by  stripping  the  blocks  from  the  foundation  and 
placing  a  new  surface  in  the  same  manner  as  the  first 
one;  with  a  board  foundation  that  also  must  be  relaid. 
Observations  made  by  Mr.  Kummer*  seem  to  indi- 
cate that  the  continual  wetting  of  a  wood  block  sur- 
face tends  to  materially  reduce  the  resistance  to  wear. 
Blocks  from  a  street  which  had  been  sprinkled  "  instead 
of  being  pounded  down  and  dense  and  hard,  as  is  the 
case  on  streets  not  so  sprinkled,  had  broomed  out  under 
the  action  of  travel  and  the  preservative  material 
mechanically  pounded  out  of  the  wood  by  the  com- 
bined action  of  the  travel  and  water.  This,  of  course, 
leaves  the  surface  of  the  block  unprotected  by  the 
antiseptic  preservative  and  subject  to  decay.  It  also, 
in  its  spongy  condition,  offers  poor  resistance  to  wear.  " 
The  surface  of  wood  block  pavement  does  not  give  off 
fine  dust,  and  need  only  be  sprinkled  sufficiently  to  be 
swept  clean.  For  the  best  results  it  is  necessary  that 
the  surface  should  be  kept  free  from  dirt. 

*  Engineering  Record,  August  25,  1906. 


CHAPTER  XL 

STONE-BLOCK  PAVEMENTS. 
ART.  80.     STONE  FOR  PAVEMENTS. 

STONE-BLOCK  pavements  are  commonly  employed 
where  the  traffic  is  heavy  and  a  material  needed  which 
will  resist  well  under  wear. 

Stone  for  this  purpose  must  possess  sufficient  hard- 
ness to  resist  the  abrasive  action  of  wheels.  It  must 
be  tough,  in  order  that  it  may  not  be  broken  by  shocks. 
It  should  be  impervious  to  moisture  and  capable  of 
resisting  the  destructive  agencies  of  the  atmosphere 
and  of  weather  changes. 

Experience  only  can  determine  the  availability  of 
any  particular  stone  for  this  use.  The  stone  may  be 
tested  in  the  same  manner  as  brick,  and  perhaps  some- 
thing predicated  as  to  the  probability  of  its  wearing 
well  under  traffic;  but  the  conditions  of  the  use  of  the 
material  in  the  pavement  are  quite  different  from  those 
under  which  it  may  be  tested,  and  any  tests  looking  to 
a  determination  of  its  weathering  properties  are  apt  to 
be  misleading. 

Examination  of  a  stone  as  to  its  structure,  the  close- 
ness of  grain,  homogeneity,  etc.,  may  assist  in  forming 
an  idea  of  its  nature  and  value  for  wear.  Observations 
of  any  surfaces  which  may  have  been  exposed  for  a 
considerable  time  to  the  weather,  either  in  structures 
or  in  the  quarry,  will  be  the  most  efficient  method  of 
forming  an  opinion  concerning  the  weathering  proper- 

328 


STONE-BLOCK  PAVEMENTS.  329 

ties  of  the  stone.  The  conditions  of  use  in  pavements 
are,  however,  somewhat  different  from  ordinary  expo- 
sure in  structures,  on  account  of  the  material  in  the 
pavement  being  subject  to  the  action  of  water  contain- 
ing acids  and  organic  substances  due  to  excretal  and 
refuse  matter.  A  low  degree  of  permeability  usually 
indicates  that  a  material  will  not  be  greatly  affected  by 
these  influences  and  also  that  the  effect  of  frost  will 
not  be  great. 

Granite  and  sandstones  are  commonly  employed  for 
paving  blocks  and  furnish  the  best  material.  Lime- 
stones are  sometimes  used,  but  have  seldom  been  found 
satisfactory.  Trap-rock  and  the  harder  granites,  while 
answering  well  the  requirements  as  to  durability  and 
resistance  to  wear,  are  objectionable  on  account  of 
their  tendency  to  wear  smooth  and  become  slippery 
and  dangerous  to  horses.  Granite  or  syenite  of  a  tough, 
homogeneous  nature  is  probably  the  best  material  for 
the  construction  of  a  durable  pavement  for  heavy 
traffic.  Granites  of  a  quartzy  nature  are  usually  brittle 
and  do  not  resist  well  under  the  blows  of  horses'  feet  or 
the  impact  of  vehicles  on  a  rough  surface.  Those  con- 
taining a  high  percentage  of  feldspar  are  likely  to  be 
affected  by  atmospheric  agencies,  while  those  in  which 
mica  predominates  wear  rapidly  on  account  of  their 
laminated  structure. 

Sandstones  of  a  close-grained,  compact  nature  often 
give  very  satisfactory  results  under  heavy  \vear.  They 
are  less  hard  than  granite  and  wear  more  rapidly,  but 
do  not  become  so  smooth  and  slippery,  and  commonly 
form  a  pavement  that  is  more  satisfacto^  from  the 
point  of  view  of  the  user.  Sandstones  differ  very 
widely  in  character,  their  value  depending  chiefly  upon 
the  nature  of  the  cementing  material  which  holds  them 


330       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

together.  In  order  that  a  stone  may  wear  well  and 
evenly  in  a  pavement  it  is  desirable  that  it  be  fine- 
grained, dense  and  homogeneous,  as  well  as  cemented 
by  a  material  which  is  not  brittle  and  is  nearly  imper- 
vious to  moisture.  Those  sandstones  in  which  the 
cementing  material  is  of  an  argillaceous  or  calcareous 
nature  are  apt  to  be  perishable  when  exposed  to  the 
weather.  The  Medina  sandstones  of  Western  New 
York  and  Ohio  have  been  quite  extensively  used  for 
paving  purposes  and  prove  a  very  satisfactory  material 
for  such  use. 

Limestone  has  not  usually  been  successful  in  use  for 
the  construction  of  block  pavements  on  account  of  its 
lack  of  durability  against  atmospheric  influences.  The 
action  of  frost  commonly  causes  weakness  and  shiver- 
ing, which  produces  uneven  and  destructive  wear  under 
traffic.  There  are,  however,  as  wide  variations  in  the 
characteristics  of  limestones  as  in  those  of  sandstones, 
and  there  may  be  possible  exceptions  to  the  rule  that, 
in  general,  limestone  is  not  a  desirable  material  for 
block  pavement. 

ART.  81.     COBBLESTONE  PAVEMENTS. 

Cobblestones  have  in  the  past  been  quite  extensively 
used  in  the  construction  of  street  pavements,  although 
at  the  present  time  they  have  been  for  the  most  part 
abandoned.  They  are  not  usually  durable  pavements 
as  the  stones  are  easily  loosened  from  their  positions, 
although  the  stones  themselves  may  be  practically 
indestructible  arid  used  again  and  again  in  reconstruct- 
ing the  surface 

Cobblestone  pavements  as  commonly  constructed 
are  also  objectionable  because  they  are  permeable  to 


STONE-BLOCK    PAVEMENTS.  331 

water  and  difficult  to  clean.  They  therefore  collect, 
and  become  saturated  with,  the  filth  of  the  street 
and  are  very  liable  to  injury  from  frost.  They  are 
also  extremely  rough  and  unsatisfactory  in  use  for 
travel. 

For  paving  the  side-gutters,  where  broken  stone  or 
sometimes  where  wood  is  used  for  the  traveled  portion 
of  the  street,  cobblestones  may  often  be  convenient 
and  useful,  and  form  a  cheap  and  satisfactory  means 
of  disposing  of  surface  drainage.  Such  an  arrange- 
ment is  shown  in  Fig.  33  (p.  365). 

Cobblestones  as  used  for  pavements  are  usually 
rounded  pebbles  from  3  to  8  inches  in  diameter.  They 
are  set  on  end  in  a  layer  of  sand  or  gravel,  rammed  into 
place  until  firml\T  held  in  position,  and  then  covered 
with  sand  or  fine  gravel  and  left  to  the  action  of  travel, 
which  soon  works  the  upper  layer  of  sand  into  the 
interstices  between  the  stones. 

ART.  82.     BELGIAN  BLOCKS. 

Belgian  block  is  the  name  commonly  applied  to  a 
pavement  formed  of  nearly  cubical  blocks  of  hard  rock. 
In  the  vicinity  of  New  York  this  pavement  has  been 
largely  used,  the  material  being  trap-rock  from  the 
valley  of  the  lower  Hudson.  The  blocks  are  usually 
from  5  to  7  inches  upon  the  edges,  with  nearly  parallel 
faces,  and  as  commonly  laid  are  placed  upon  a  founda- 
tion layer  of  sand  or  gravel  about  6  inches  thick.  This 
shape  of  block, is  objectionable  on  account  of  the  width 
between  joints  being  too  great  to  afford  good  foothold 
to  horses.  The  materials  of  which  Belgian  blocks  have 
ordinarily  been  formed  are  very  hard  and  (as  already 
noted  in  Art.  81)  wear  smooth  in  service,  becoming 


332        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

slippery  and  thus  increasing  the  effect  of  the  too  wide 
block.  It  is  also  better  to  have  the  length  of  the 
blocks  somewhat  greater  across  the  street  and  let  them 
break  joints  in  that  direction  in  order  that  they  may 
give  greater  resistance  to  displacement  under  passing 
wheel-loads. 

The  older  pavements  of  this  character  were  usually 
placed  upon  a  sand  foundation.  More  recently  this 
practice  has,  in  the  better  class  of  work,  been  super- 
seded by  a  more  solid  construction,  a  concrete  base 
being  used. 

These  pavements  are  now  very  little  used,  having 
given  place  to  granite  or  sandstone  blocks. 

ART.  83.     GRANITE  AND  SANDSTONE  BLOCKS. 

For  the  construction  of  the  better  class  of  stone- 
block  pavements,  blocks  of  tough  granite  or  sandstone 
are  used,  set,  in  the  best  work,  upon  a  concrete  base, 
although  sometimes  placed  upon  a  foundation  of  sand 
or  gravel. 

These  pavements  when  well  constructed  are  about 
the  most  satisfactory  means  yet  devised  for  providing 
for  very  heavy  traffic,  as  they  present  a  maximum 
resistance  to  wear  with  a  fairly  good  foothold  for  horses, 
and  are  much  more  agreeable  in  service  than  the 
old  form  of  rough  pavements.  There  is  still  much  to 
be  desired  in  the  attainment  of  smoothness  and  ab- 
sence of  noise,  and,  as  a  general  thing,  it  may  be  said 
that  pavements  of  this  kind  are  desirable  only  where 
the  weight  of  traffic  is  so  great  that  the  smoother  pave- 
ments would  not  offer  sufficient  resistance  to  wear. 
Even  in  such  cases  it  may  frequently  be  questionable 
whether  an  additional  expense  for  maintaining  a  pave- 


STONE-BLOCK  PAVEMENTS.  333 

ment  which  would  be  more  pleasant  in  use  and  less 
objectionable  to  occupants  of  adjoining  premises  would 
not  be  advisable  from  an  economical  as  well  as  from 
an  aesthetic  point  of  view. 

Blocks  for  stone  pavements,  in  the  best  work,  are  cut 
in  the  form  of  parallelepipeds,  9  to  12  inches  long,  3 
inches  wide,  and  6  or  7  inches  deep.  The  length  should 
be  sufficient  to  permit  the  blocks  to  break  joints  across 
the  street.  The  width  should  be  less  than  that  of  a 
horse's  hoof  in  order  that  the  joints  in  the  direction  of 
travel  may  be  close  enough  together  to  prevent  a  horse 
from  slipping  in  getting  a  foothold.  The  depth  should 
be  sufficient  to  give  a  bearing  surface  in  the  joints 
large  enough  to  prevent  the  blocks  from  tipping  when 
the  load  comes  upon  one  end  of  them. 

Specifications  for  granite  blocks  in  New  York  City 
in  1908  are  as  follows: 

"The  blocks  to  be  used  shall  be  of  a  durable,  sound 
and  uniform  quality  of  granite,  each  stone  measuring 
not  less  than  eight  (8)  inches,  nor  more  than  twelve  (12) 
inches  in  length;  not  less  than  three  and  one-half  (3^) 
nor  more  than  four  and  one-half  (4^)  inches  in  width, 
and  not  less  than  seven  (7)  nor  more  than  eight  (8) 
inches  in  depth,  and  the  stones  shall  be  of  the  same 
quality  as  to  hardness,  color  and  grain.  No  outcrop, 
soft,  brittle  or  laminated  stone  will  be  accepted.  The 
blocks  are  to  be  rectangular  on  top  and  sides,  uniform 
in  thickness,  to  lay  closel3T,  and  with  fair  and  true 
surfaces,  free  from  bunches.  Over  special  construc- 
tions, the  blocks  may  be  of  dimensions  other  than 
above  specified  when  approved  by  the  Engineer.  The 
stone  from  each  quarry  shall  be  piled  and  laid  separately 
in  different  sections  of  the  work,  and  in  no  case  shall  the 
stones  from  different  quarries  be  mixed. " 


334        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  84.    CONSTRUCTION  OF  STONE-BLOCK 
PAVEMENTS. 

Stone-block  pavement  for  durable  and  effective 
service  should  be  placed  upon  very  firm  foundations. 
Bases  of  concrete  are  usually  employed  and  give  the 
best  results.  These  foundations  are  formed  as  described 
in  Art.  57.  and  consist  of  a  layer  of  concrete  4  to  8 
inches  thick,  6  inches  being  the  most  common  depth. 

In  constructing  the  pavement,  a  cushion  coat  of  sand, 
usually  I  to  2  inches  thick,  is  spread  upon  the  base  of 
concrete  for  the  purpose  of  allowing  the  bases  of  the 
paving  blocks  to  be  firmly  bedded  when  the  tops  are 
brought  to  an  even  surface,  the  sand  readily  adjusting 
itself  so  as  to  fill  all  the  spaces  beneath  the  blocks  and 
to  offer  a  uniform  resistance  to  downward  motion  in 
every  part  of  the  pavement,  and  in  like  manner  trans- 
mitting the  loads  which  come  upon  the  pavement  to 
the  foundation  so  as  to  evenly  distribute  them  over  the 
surface  of  the  concrete.  The  sand  used  for  this  pur- 
pose should  be  clean  and  dry,  and  all  large  particles 
sifted  out,  as  they  may  prevent  the  blocks  adjusting 
themselves  properly.  A  thin  laj^er  of  asphaltic  cement 
is  sometimes  used  in  place  of  the  sand  with  very  good 
results. 

The  blocks  should  be  laid  as  close  together  as  pos- 
sible in  order  to  make  the  joints  small.  They  are  laid, 
like  brick,  with  the  longest  dimension  across  the  street, 
and  arranged  in  courses  transverse  to  the  street,  with 
the  stone  in  consecutive  courses  breaking  joints. 

After  the  blocks  are  placed  they  are  well  rammed  to 
a  firm  unyielding  bearing  and  an  even  surface.  Stones 
that  sink  too  low  under  the  ramming  must  be  taken 
out  and  raised  by  putting  more  sand  underneath. 


STONE-BLOCK  PAVEMENTS.  335 

As  in  the  case  of  other  block  pavements,  those  of 
stone  should  be  made  as  impervious  to  moisture  as 
possible.  The  foundation  should  be  kept  dry,  and 
moisture  prevented  from  penetrating  beneath  the 
blocks  where  it  has  a  tendency  to  cause  unequal  settle- 
ment under  loads  or  disruptions  under  the  action  of 
frost.  In  the  better  class  of  work,  therefore,  the  joints 
are  filled  with  an  impervious  material  which  cements 
the  blocks  together.  Asphalt  or  coal-tar  paving  cement 
is  commonly  employed  for  this  purpose,  as  with  brick 
and  wood,  and  seems  the  most  satisfactory  in  use, 
although  hydraulic  cement  mortar  is  sometimes  used. 
The  coal-tar  cement  is  commonly  made  by  mixing 
coal-tar  pitch  with  gas-tar  and  oil  of  creosote,  a  pro- 
portion sometimes  employed  being  100  pounds  pitch, 
4  gallons  tar,  and  I  gallon  creosote. 

The  use  of  cement  between  the  blocks  binds  them 
together  and  increases  the  strength  of  the  pavement  as 
well  as  the  resistance  of  the  blocks  to  being  forced  out 
of  surface.  It  also  deadens  to  some  extent  the  noise 
from  the  passing  of  vehicles  where  asphaltic  or  coal-tar 
cement  is  used. 

A  method  commonly  used  for  filling  the  joints  is  to 
first  fill  them  about  one  third  full  of  small  gravel,  then 
pour  in  the  paving  cement  until  it  stands  above  the 
gravel;  then  another  third  full  of  gravel,  more  cement 
as  before;  then  gravel  to  a  little  below  the  top,  and  the 
joint  filled  full  of  cement;  after  which  a  coating  of 
fine  gravel  is  distributed  over  the  surface. 

Sometimes  the  joints  are  filled  with  gravel  before  the 
blocks  are  rammed  to  surface,  and  the  paving  cement 
afterward  poured  into  the  joints.  This  has  the  advan- 
tage of  bringing  the  blocks  to  a  very  firm  bearing,  and 
secures  complete  filling  of  the  joints. 


336        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Various  modifications  of  the  method  above  outlined 
are  used  in  the  principal  cities  for  a  pavement  to  with- 
stand heaviest  traffic  and  secure  a  maximum  of  dura- 
bility; essentially  it  represents  the  best  modern  practice. 
The  specifications  used  in  New  York  City  in  1908 
contain  the  following  requirements: 

"  29.  On  the  concrete  foundation,  as  designated,  shall 
be  laid  a  bed  of  clean,  coarse,  dry  sand  to  such  depth 
(in  no  case  less  than  one  and  a  half  [ij]  inches)  as  may 
be  necessary  to  bring  the  surface  of  the  pavement, 
when  thoroughly  rammed,  to  the  proper  grade. 

"On  this  sand  bed,  and  to  the  grade  and  crown 
specified,  shall  be  laid  the  stone  blocks  at  right  angles 
to  the  line  of  the  street  or  at  such  angle  as  may  be 
directed.  Each  course  of  blocks  shall  be  laid  straight 
and  regularly,  with  the  end  joints  by  a  lap  of  at  least 
three  (3)  inches,  and  in  no  case  shall  stone  of  different 
width  be  laid  in  the  same  course  except  on  curbs.  All 
joints  shall  be  close  joints  except  that  when  gravel 
filling  is  used,  the  joints  between  courses  shall  be  not 
more  than  three-quarters  (f)  of  an  inch  in  wudth. 

"After  the  blocks  are  laid  on  a  concrete  foundation, 
they  shall  be  covered  with  a  clean,  hard  and  dry  gravel, 
which  shall  have  been  artificially  heated  and  dried  in 
proper  appliances,  placed  in  close  proximity  to  the 
work,  the  gravel  to  be  brushed  in  until  all  the  joints  are 
filled  therewith  to  within  three  (3)  inches  of  the  top. 
The  gravel  must  be  washed  white  quartz  and  be  entirely 
free  from  sand  or  dirt,  and  must  have  passed  through 
a  sieve  of  five-eighths  (f )  inch  mesh  and  been  retained 
by  a  three-eighths  (f)  inch  mesh. 

"The  blocks  must  then  be  thoroughly  rammed  and 
the  ramming  repeated  until  they  are  brought  to  an 
unyielding  bearing  with  a  uniform  surface,  true  to  the 


STONE-BLOCK  PAVEMENTS.  337 

given  grade  and  crown.  No  ramming  shall  be  done 
within  twenty  (20)  feet  of  the  face  of  the  work  that  is 
being  laid. 

"  The  boiling  paving  cement,  heated  to  a  temperature 
of  300°  F.and  of  the  composition  hereinbefore  described, 
shall  then  be  poured  into  the  joints  until  the  same  are 
full,  and  remain  full  to  the  top  of  the  gravel.  Hot 
gravel  shall  then  be  poured  along  the  joints  until  they 
are  full  flush  with  the  top  of  the  blocks,  when  they 
shall  again  be  poured  with  the  paving  cement  till  all 
voids  are  completely  filled/' 

ART.  85.     STONE  TRACKWAYS. 

In  some  of  the  European  cities,  particularly  in  Italy, 
stone  trackways  are  sometimes  employed  on  streets  of 
heavy  traffic  for  the  purpose  of  diminishing  traction. 
These  trackways  are  formed  of  smooth  blocks  of  stone 


FIG.  27. 

4  to  6  feet  long,  1 8  to  24  inches  wide,  and  6  to  8  inches 
deep,  laid  flat  and  end  to  end  so  as  to  form  a  smooth 
surface  upon  which  wheels  may  move  with  the  least 
possible  resistance.  Between  the  tracks,  and  usually 
the  remainder  of  the  street,  is  commonly  paved  with 
cobble.  The  method  of  construction  is  shown  in  Fig. 
27.  The  tracks  drain  to  the  middle,  and  the  pavement 
between  is  made  concave  and  provided  with  openings 
into  the  storm  sewers  for  the  escape  of  surface-water. 
The  track  and  pavement  are  laid  upon  a  layer  of  sand 
resting  upon  a  broken-stone  or  gravel  foundation. 


338       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Such  trackways  are  quite  durable  under  heavy  traffic, 
and  give  light  tractive  resistance.  They  are  not,  how- 
ever, desirable  on  the  streets  of  towns  where  smooth 
pavements  might  be  used,  and  are  too  expensive  for 
use  on  country  roads. 

Steel  trackways  have  frequently  been  proposed,  and 
in  a  few  instances  have  been  tried,  but  have  not  been 
found  successful  and  do  not  seem  likely  to  become  of 
any  considerable  importance. 


CHAPTER  XII. 
CONCRETE    PAVEMENTS. 

ART.  86.    CONCRETE  AS  SURFACE  MATERIAL. 

THE  use  of  Portland  cement  concrete  as  material  for 
the  surfaces  of  street  pavements  has  been  in  use  in  a 
small  way  since  about  1895.  Until  after  1900,  however, 
these  pavements  were  very  few  in  number  and  regarded 
as  rather  doubtful  experiments.  Since  1900  there  has 
been  a  considerable  increase  in  the  use  of  this  material, 
and  quite  a  number  of  cities  have  tried  it  to  some  extent. 
Most  of  the  work  that  has  been  done  is  of  too  recent  date 
to  show  final  results,  or  determine  the  best  methods  of 
construction.  In  several  instances,  the  early  pavements 
have  given  good  wear  under  moderate  traffic,  but  more 
experience  is  necessary  to  determine  the  extent  to  which 
these  materials  may  meet  the  requirements  of  more  general 
use,  and  to  formulate  methods  of  construction  to  secure 
the  best  results. 

The  objects  in  most  instances  of  engineers  who  have 
constructed  pavements  of  this  kind  have  been  to  secure 
pavements  for  moderate  or  light  traffic  at  less  cost  than 
brick,  or  other  satisfactory  pavements,  could  be  con- 
structed. Several  methods  of  construction  have  been 
patented,  and  many,  if  not  most,  of  the  concrete  pave- 
ments now  in  use  have  been  constructed  under  some  of 
these  patents.  These  refer  both  to  the  composition  of 
the  cement  mortar  or  concrete  employed  for  surfacing 
the  pavement,  and  to  the  method  of  construction. 

339 


340      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

Three  types  of  construction  have  been  used  for  pave- 
ments of  this  class: 

(a)  Mortar-surfaced  pavements,  in  which  a  surfacing 
of  mortar  is  applied  to  an  ordinary  concrete  foundation 
before  the  concrete  has  set,  in  order  that  adhesion  may 
develop  between  the  mortar  and  concrete  in  setting. 

(b)  Monolithic    concrete     pavements,     which     consist 
of  a  single  layer  of  concrete  of  the  full  thickness  of  the 
pavement. 

(c)  Grouted  concrete  pavements,   constructed  by  first 
placing  the  coarse  aggregate  to  the  required  thickness, 
and    then  pouring  a  grout  of  cement  mortar  over  the 
surface,  so  as  to  fill  the  voids  in  the  aggregate,  which  h 
rolled  to    a  firm  surface  either  before  or  after  the  grout 
is  applied. 

Concrete  pavements  seem  to  give  promise  of  consider- 
able development  in  the  immediate  future,  and  it  is  prob- 
able that  their  use  will  rapidly  extend. 

ART.  87.    PORTLAND  CEMENT. 

Portland  cement  is  manufactured  by  burning  a  mixture 
of  limestone  and  clay,  shale  or  other  argillaceous  mate- 
rial; the  mixture  being  accurately  proportioned  to  give 
correct  relations  between  the  percentages  of  lime,  silica 
and  alumina  in  the  resulting  cement.  The  materials 
are  finely  ground,  so  as  to  obtain  an  intimate  mixture, 
and  the  temperature  of  burning  carefully  regulated  to 
secure  proper  chemical  combinations.  The  clinker  ob- 
tained from  the  burning  of  these  materials  is  then  finely 
ground  into  the  powder  which  is  used  as  cement. 

The  manufacture  of  Portland  cement  is  very  exten- 
sively carried  on  throughout  the  United  States,  and 
cement  of  high  grade  can  be  obtained  with  little  difficulty 


CONCRETE  PAVEMENTS.  341 

in  nearly  any  locality.  It  is  important  that  only  first- 
class  cement  should  be  used  in  the  construction  of  paved 
surfaces,  and  care  should  always  be  taken  in  the  selec- 
tion of  the  cement.  The  requirements  pf  this  service 
are  very  severe,  and  the  soundness  of  the  cement  is  of 
special  importance.  When  it  is  not  feasible  to  make 
tests  of  the  cement,  considerable  reliance  may  be  placed 
upon  the  selection  of  a  good  brand,  but  usually  tests  are 
imposed  to  insure  proper  quality. 

The  tests  upon  which  dependence  is  placed  to  form 
judgment  of  the  quality  of  cement  are  those  for  tensile 
strength,  fineness  and  soundness.  The  standard  methods 
of  testing  cement,  which  have  been  recommended  by  the 
American  Society  of  Civil  Engineers  are  now  commonly 
employed,  and  standard  specification  requirements  rec- 
ommended by  the  American  Society  for  Testing  Materials 
are  very  generally  followed.  These  requirements  are 
as  follows: 

PORTLAND  CEMENT. 

"  Definition.  This  term  is  applied  to  the  finely 
pulverized  product  resulting  from  the  calcination  to 
incipient  fusion  of  an  intimate  mixture  of  properly  pro- 
portioned argillaceous  and  calcareous  materials,  and 
to  which  no  addition  greater  than  3  per  cent  has  been 
made  subsequent  to  calcination. 

SPECIFIC  GRAVITY. 

"  The  specific  gravity  of  cement  shall  not  be  less  than 
3.10.  Should  the  test  of  cement  as  received  fall  below 
this  requirement,  a  second  test  may  be  made  upon  a 
sample  ignited  at  a  low  red  heat.  The  loss  in  weight 
of  the  ignited  cement  shall  not  exceed  4  per  cent. 


342      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 
FINENESS. 

"  It  shall  leave  by  weight  a  residue  of  not  more  than 
8  per  cent  on  the  No.  100,  and  not  more  than  25  per  cent 
on  the  No.  200  sieve. 


TIME  OF  SETTING. 

"  It  shall  not  develop  initial  set  in  less  than  30  minutes; 
and  must  develop  hard  set  in  not  less  than  one  hour, 
nor  more  than  10  hours. 


TENSILE  STRENGTH. 

'  The  minimum  requirements  for  tensile  strength  for 
briquettes  one  square  inch  in  cross-section  shall  be  as 
follows,  and  the  cement  shall  show  no  retrogression  in 
strength  within  the  periods  specified: 

NEAT  CEMENT. 

Age.  Strength. 

24  hours  in  moist  air 175  Ibs. 

7  days  (i  day  in  moist  air,    6  days  in  water) 500    ' ' 

28  days  (i  day  in  moist  air,  27  days  in  water) 600    " 

ONE  PART  CEMENT,  THREE  PARTS  STANDARD  OTTAWA  SAND. 

7  days  (i  day  in  moist  air,    6  days  in  water) 200  Jbs. 

28  days  (i  day  in  moist  air,  27  days  in  water) 275    " 

CONSTANCY  OF  VOLUME. 

"  Pats  of  neat  cement  about  3  inches  in  diameter, 
\  inch  thick  at  the  center,  and  tapering  to  a  thin  edge, 
shall  be  kept  in  moist  air  for  a  period  of  24  hours. 

"  (a)  A  pat  is  then  kept  in  air  at  normal  temperature 
and  observed  at  intervals  for  at  least  28  days. 


CONCRETE  PAVEMENTS.  343 

"  (b)  Another  pat  is  kept  in  water  maintained  as 
near  70°  F.  as  practicable,  and  observed  at  intervals  for 
at  least  28  days. 

"  (c)  A  third  pat  is  exposed  in  any  convenient  way  in 
an  atmosphere  of  steam,  above  boiling  water,  in  a  loosely 
closed  vessel  for  5  hours. 

"  These  pats,  to  satisfactorily  pass  the  requirements, 
shall  remain  firm  and  hard,  and  show  no  signs  of  dis- 
tortion, checking  cracking  or  disintegrating. 

SULPHURIC  ACID  AND  MAGNESIA. 

"  The  cement  shall  not  contain  more  than  1.75  per 
cent  of  anhydrous  sulphuric  acid  (SOs),  nor  more  than  4 
per  cent  of  magnesia  (MgO)." 

Portland  cement  is  sold  packed  in  barrels,  containing 
about  376  pounds  of  cement,  or  in  canvas  bags,  each 
containing  one-fourth  barrel.  For  the  purpose  of  pro- 
portioning mortar  or  concrete  by  volume  a  barrel  is  com- 
monly taken  as  holding  3.8  cubic  feet,  or  the  cement  is 
assumed  to  weigh  about  100  pounds  per  cubic  foot. 
Cement  must  always  be  stored  in  a  dry  place  and  care- 
fully protected  from  moisture;  it  should  not  be  piled  in 
contact  with  the  ground,  and  must  be  covered  to  exclude 
all  water. 

ART.  88.    PORTLAND  CEMENT  MORTAR. 

Portland  cement  mortar  is  formed  by  mixing  Portland 
cement  with  a  fine  mineral  aggregate,  and  wetting  the 
mixture  to  a  paste,  which  hardens  with  the  setting  of  the 
cement.  In  the  preparation  of  cement  mortar,  care 
must  be  used  to  insure  that  the  cement  and  aggregate 
are  thoroughly  mixed  and  evenly  distributed  through  the 
mortar.  When  mixing  by  hand,  the  dry  materials  should 


344      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

be  mixed  to  a  uniform  color  before  adding  the  water. 
The  mortar  must  in  all  cases  be  placed  in  the  work 
before  beginning  to  set,  and  be  left  undisturbed  until 
thoroughly  set,  and  until  it  has  acquired  sufficient  strength 
to  resist  distortion. 

Fine  Aggregate.  The  fine  aggregate  is  either  sand, 
stone  screenings  or  other  similar  material,  which  will 
pass  a  screen  of  J-inch  mesh.  Sand  for  this  use  should 
be  coarse  or  contain  a  considerable  percentage  of  coarse 
grains  (not  passing  a  sieve  of  thirty  meshes  to  the  inch). 
It  should  be  clean  and  free  from  loam,  and  other  impur* 
ities,  and  should  not  contain  more  than  from  3  per  cent 
to  6  per  cent  of  dust  which  will  pass  a  sieve  of  100  meshes 
to  the  inch. 

In  preparing  mortar,  good  results  require  that  the  voids 
in  the  fine  aggregate  should  be  completely  filled  with 
cement,  and  the  amount  of  cement  used  should  be 
somewhat  larger  than  the  volume  of  voids  to  be  filled. 
It  is  therefore  desirable  that  the  sizes  of  the  particles  of 
fine  aggregate  be  graded  so  as  to  give  a  minimum  of 
voids,  and,  in  important  work  it  is  necessary  to  carefully 
examine  the  aggregate  in  this  particular  and  proportion 
the  mortar  accordingly.  The  following  tests  for  deter- 
mining the  voids  in  aggregates,  recommended  by  a  com- 
mittee of  the  National  Association  of  Cement  Users, 
may  be  used  with  advantage  for  this  purpose: 

Test  for  Voids.  To  determine  the  voids  in  the  coarse 
aggregate  or  fine  aggregate:  Prepare  a  vessel,  the  cubical 
contents  of  which  is  exactly  one  cubic  foot  (1728  cubic 
inches),  being  smaller  at  the  top  than  at  the  bottom. 
Fill  the  vessel  with  the  aggregate,  thoroughly  dried, 
'  coarse  '  or  '  fine  '  as  the  case  may  be,  which  is  to  be  used. 
Shake  or  jar  the  vessel  containing  the  aggregate  until  it  is 
compacted  as  thoroughly  as  possible  and  the  vessel  is 


CONCRETE  PAVEMENTS.  345 

level  full.  Then  ascertain  the  net  weight  of  the  fine 
aggregate  in  the  vessel,  deduct  this  weight  from  w  (the 
weight  of  one  foot  cube  of  mineral  of  which  the  fine 
aggregate  is  composed)  divide  the  difference  thus  obtained 
by  w.  The  result  is  the  percentage  of  voids. 

When  mortar  surfaces  are  employed  upon  pavements, 
the  fine  aggregates  are  frequently  composed  of  crushed 
granite  screenings,  or  sometimes  of  special  mixtures  of 
granite  and  sand,  or  limestone,  intended  to  give  effective 
resistance  to  wear,  or  to  render  the  paving  surface  gritty 
and  prevent  it  being  .slippery.  Some  of  these  com- 
binations of  materials  are  patented.  For  the  purpose 
of  comparing  the  values  of  various  fine  aggregates  for 
use  in  mortars,  the  Committee  of  the  National  Associa- 
tion of  Cement  Users  recommends  that  they  be  tested 
in  comparison  with  standard  sand,  as  follows: 

"  Mortars  composed  of  i  part  Portland  cement  and 
3  parts  fine  aggregate  by  weight  when  made  into  briquettes 
should  show  a  tensile  strength  of  at  least  70  per  cent  of 
the  strength  of  i :  3  mortar  of  the  same  consistency  made 
with  the  same  cement  and  standard  Ottawa  sand.  To 
avoid  the  removal  of  any  coating  on  the  grains  which 
may  affect  the  strength,  bank  sands  should  not  be  dried 
before  being  made  into  mortar,  but  should  contain 
natural  moisture.  The  percentage  of  moisture  may  be 
determined  upon  a  separate  sample  for  correcting  weight 
of  sand.  From  10  to  40  per  cent  more  water  may  be 
required  in  mixing  bank  or  artificial  sands  than  for 
standard  Ottawa  sand  to  produce  the  same  consistency." 

ART.  89.    PORTLAND  CEMENT  CONCRETE. 

Portland  cement  concrete  consists  of  a  mixture  of 
Portland  cement,  or  Portland  cement  mortar,  with  a 


346      A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

coarse  aggregate  (gravel,  broken  stone  or  similar  material), 
the  materials  being  so  proportioned  as  that  the  mortar 
fills  the  voids  in  the  coarse  aggregate,  and  the  cement 
those  of  the  fine  aggregate. 

Coarse  Aggregate.  The  coarse  aggregate  usually  con- 
sists of  gravel  or  crushed  rock,  all  of  which  is  retained  by 
a  sieve  of  J-inch  meshes,  and  grading  upwards  to  any 
size  which  may  be  readily  handled  in  mixing  and  not 
too  large  to  pack  in  the  thickness  of  the  concrete  layer. 
The  largest  stones  should  not  be  greater  in  diameter  than 
f  the  depth  of  the  concrete  layer,  and  a  grading  of  sizes 
is  desirable,  which  will  reduce  the  voids  to  be  filled  with 
mortar  to  a  minimum.  In  some  instances,  the  aggregate 
is  screened  into  several  sizes,  and  these  mixed  in  such 
proportions  as  to  give  a  minimum  of  voids. 

When  the  concrete  is  to  be  used  as  surface  material 
care  should  be  taken  to  secure  a  uniform  mixture,  in 
order  that  the  resistance  to  wear  may  be  even.  Some 
engineers  specify  nearly  uniform  sizes  of  rock  on  this 
account,  using  a  larger  proportion  of  mortar.  Commonly, 
however,  crusher-run  rock  is  used,  with  only  the  fine 
parts  and  the  pieces  which  are  too  large  screened  out, 
no  grading  of  sizes  being  attempted  on  account  of  cost. 

The  material  used  for  the  coarse  aggregate  should  be 
of  hard  and  durable  character.  Gravels,  limestones 
and  trap  rocks  are  commonly  employed.  For  the  mono- 
lithic pavement,  where  the  concrete  forms  the  wearing 
surface  of  the  pavement,  particular  attention  should 
be  given  to  securing  rock  that  will  be  resistant  to  wear. 
For  such  use,  special  mixtures  of  rock  are  sometimes 
made,  two  kinds  of  rocks  of  somewhat  different  wearing 
properties  being  used  in  fixed  proportions.  Some  of 
these  mixtures  are  patented.  In  general  the  choice  of 
stone  is  limited  by  what  may  be  available  in  the  neigh- 


CONCRETE   PAVEMENTS.  347 

» 

borhood  of  the  work,  but  careful  consideration  should 
always  be  given  to  the  suitability  of  the  material.  When 
the  concrete  is  for  foundation  only  and  not  exposed  upon 
the  surface,  a  poorer  grade  of  rock  may  be  employed. 
The  Association  for  Standardizing  Paving  Specifica- 
tions has  suggested  the  following  specification  for  con- 
crete for  use  in  pavements; 

FINE  AGGREGATE. 

"  The  fine  aggregate  shall  consist  of  any  material  of 
siliceous,  granite  or  igneous  origin,  free  from  mica  in 
excess  of  5  per  cent,  and  other  impurities,  uniformly 
graded,  the  particles  ranging  in  size  from  }  inch  down  to 
that  which  will  pass  a  No.  100  standard  sieve. 

COARSE  AGGREGATE. 

"  The  coarse  aggregate  shall  be  sound  broken  stone, 
trap  rock,  or  granite  having  a  specific  gravity  of  not  less 
than  2.6.  It  shall  be  free  from  all  foreign  matter,  uni- 
formly graded  and  shall  range  in  size  from  J  inch  up, 
the  largest  particles  not  to  exceed  in  any  dimension  one- 
half  the  thickness  of  the  concrete  in  place. 

PROPORTIONS. 

"  In  preparing  the  concrete  the  cement  and  aggregate 
shall  be  measured  separately,  and  then  mixed  in  such 
proportions  that  the  resulting  concrete  shall  contain 
fine  aggregate  amounting  to  one-half  of  the  volume  of 
the  coarse  aggregate,  and  that  five  cubic  feet  of  concrete 
in  place  will  contain  ninety-four  (94)  pounds  of  cement. 


A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 


MIXING. 

"  The  ingredients  of  the  concrete  shall  be  thoroughly 
mixed,  sufficient  water  being  added  to  obtain  the  desired 
consistency,  and  the  mixing  continued  until  the  materials 
are  uniformly  distributed  and  each  particle  of  the  fine 
aggregate  is  thoroughly  coated  with  cement,  and  each 
particle  of  the  coarse  aggregate  is  thoroughly  coated  with 
mortar. 

"  Where  a  mechanical  mixer  is  used,  the  materials  must 
be  proportioned  dry,  and  then  deposited  in  the  mixer 
all  at  the  same  time.  The  mixer  must  produce  a  con- 
crete of  uniform  consistency  and  color,  with  the  stones 
thoroughly  mixed  with  water,  sand  and  cement. 

CONSISTENCY. 

"  The  materials  shall  be  mixed  wet  enough  to  produce 
a  concrete  of  a  consistency  that  will  flush  readily  under 
light  tamping,  but  which  can  be  handled  without  causing 
a  separation  of  the  coarse  aggregate  from  the  mortar. 

RE  -TEMPERING. 

"  Re-tempering,  that  is,  re-mixing  with  additional 
water,  mortar  or  concrete,  that  has  partially  hardened, 
will  not  be  permitted." 

ART.  90.       MORTAR-SURIACED  PAVEMENTS. 

Most  of  the  earlier  concrete  pavements  are  of  this 
type.  They  consist  of  a  concrete  foundation,  constructed 
in  about  the  same  manner  as  for  a  brick  or  asphalt  sur- 
face, with  a  layer  of  carefully  prepared  cement  mortar 


CONCRETE  PAVEMENTS.  349 

to  take  the  wear  of  the  traffic.  In  many  of  the  pavements, 
the  method  of  construction  is  about  the  same  as  that 
used  in  placing  cement  sidewalks,  the  depth  being  usually 
somewhat  greater,  with  a  difference  in  finishing  the  sur- 
face to  prevent  slipperiness. 

The  fine  aggregate  used  in  making  mortar  for  this 
purpose  should  be  of  hard  material,  capable  of  resisting 
abrasion.  Crushed  granite  and  trap  rock  have  fre- 
quently been  employed  for  the  purpose,  and  mixtures 
of  granite  and  limestone,  or  natural  sand  and  crushed 
limestone,  are  sometimes  used.  Fairly  good  results 
have  been  obtained  with  all  of  these  materials.  These 
pavements  are  commonly  from  5  to  7  inches  in  depth, 
with  a  surface  layer  ij  or  2  inches  thick.  The  mortar 
for  the  surface  must  always  be  placed  before  the  founda- 
tion concrete  has  set  in  order  to  secure  proper  adhesion 
between  the  two. 

In  finishing  the  surfaces  of  these  pavements,  some 
method,  intended  to  prevent  the  surface  being  too  smooth 
and  slippery,  has  usually  been  adopted.  Sometimes  the 
surface  is  cut  into  small  blocks,  tool  cuts  are  made  across 
the  street  a  few  inches  apart,  or  the  surface  is  pitted  with  a 
brass  roller.  In  other  instances,  brushing  the  surface 
with  a  stiff  broom  has  seemed  to  leave  the  surface  in  good 
condition  for  wear,  while  the  use  of  aggregate  composed 
of  a  mixture  of  two  materials  of  somewhat  different 
wearing  qualities  has  been  claimed  to  assist  in  prevent- 
ing the  pavement  being  slippery. 

Expansion  joints  along  the  curb,  or  between  the  gutter 
and  roadway  proper,  and  also  extending  across  the  street 
at  frequent  intervals  are  needed  to  prevent  the  cracking 
of  the  pavement  with  the  expansion  and  contraction  of 
the  pavement.  These  joints  extend  through  the  pave- 
ment and  are  usually  filled  with  bituminous  cement 


350      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

filler,  although  creosoted  wood  blocks  are  being  used  to 
some  extent  for  the  transverse  joints.  In  the  earlier 
pavements  expansion  was  taken  care  of  by  dividing  the 
pavement  into  rectangular  blocks,  as  in  the  construction 
of  sidewalks,  but  it  was  found  that  the  wear  of  the  pave- 
ments was  mainly  due  to  the  breaking  down  at  these 
joints,  and  particularly  the  longitudinal  joints,  and  later 
practice  has  for  the  most  part  abandoned  all  continuous 
longitudinal  joints,  except  at  the  sides.  A  method  of 
construction  which  has  been  somewhat  extensively  used 
for  this  type  of  pavement,  is  patented  and  is  known  as 
the  Blome  Granitoid  Pavement.  Specifications  some- 
times used  in  the  construction  of  surface  of  this  pavement 
are  as  follows: 

MIXING  AND  LAYING  OF  CONCRETE  AND  FORMATION 
OF  THE  BLOME  COMPANY  GRANITOID  BLOCKING. 

The  concrete  and  blocking  hereinafter  specified  shall 
be  constructed  and  manipulated  according  to  the  Blome 
Company  patents  and  processes,  using  materials  mixed 
in  the  proportions  and  laid  as  herinafter  specified. 

The  pavement  shall  consist  of  5^  inches  of  concrete, 
and  surface  blocking  if  inches,  making  a  total  of  7  inches, 
exclusive  of  foundation. 

After  the  sub-grade  and  foundation  have  been  prepared 
as  hereinbefore  specified,  there  shall  be  deposited  con- 
crete composed  of  i  part  of  Portland  cement,  3  parts  sand, 
and  4  parts  of  crushed  limestone,  trap  rock,  or  clean 
gravel.  These  materials  to  comply  with  the  require- 
ments hereinbefore  set  forth  and  shall  be  mixed  by  special 
mixing  machine  suitable  for  the  purpose  to  be  approved 
by  the  engineer  and  shall  be  mixed  at  least  five  times 
before  being  removed  from  the  mixer.  The  concrete 


CONCRETE  PAVEMENTS.  351 

shall  be  thoroughly  tamped  in  place,  and  shall  be  5^ 
inches  thick,  uniformly  at  all  points,  after  having  been 
compacted,  shall  be  laid  in  sections  with  expansion 
joints,  all  as  per  the  Blome  Company  patents  and  shall 
follow  the  slopes  of  the  finished  pavement  so  that  the 
surface  blocking  is  and  shall  be  uniformly  of  the  same 
thickness  at  all  points. 

Surfacing  Material.  After  the  concrete  has  been 
placed  and  before  it  has  begun  to  set,  there  shall  be  imme- 
diately deposited  thereon  the  Granitoid  Blocking  which 
shall  be  1}  inches  in  thickness  to  be  composed  of  two 
parts  of  the  hereinbefore  specified  Portland  cement  and 
three  parts  of  clean,  crushed  granite,  trap  rock,  hard 
stone,  crushed  gravel,  crushed  boulders,  or  other  sim- 
ilarily  hard  materials  shall  be  screened  with  all  the  dust 
removed  therefrom,  utilizing  the  following  composition 
of  this  material. 

Fifty  per  cent  of  the  granite,  trap  rock,  hard  stone, 
crushed  gravel,  crushed  boulders  or  other  similarly  hard 
materials  to  be  what  is  known  as  -J-inch  size,  30  per  cent 
of  the  J-inch  size,  and  20  per  cent  of  the  j^-'nch  size 
with  all  finer  particles  removed.  These  proportions 
of  sizes  are  extremely  essential  and  must  be  kept  abso- 
lutely accurate  as  in  this  lies  one  of  the  essential  require- 
ments to  produce  proper  results.  This  material  to  be 
mixed  with  cement  thoroughly  and  after  being  wetted 
to  a  proper  consistency  and  deposited  on  the  concrete 
shall  be  worked  into  brick  shapes  of  approximately  4^ 
inches  by  9  inches  with  rectangular  surface  similar  to 
paving  blocks,  all  as  per  special  method  and  utilizing 
grooving  apparatus  as  employed  under  the  Blome  Com- 
pany patents.  The  pavement  shall  be  sloped  in  a  manner 
as  required  by  the  city  engineer. 

Should  there  by  any  part  or  parts  of  this  pavement 


352       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

when  completed  where  the  slopes,  contours,  etc.,  have 
not  been  carried  out  in  true  manner  then  under  this 
specification  the  contractor  will  be  required  to  take  up 
such  part  or  parts  down  to  the  foundation  and  replace 
same  to  the  proper  level  without  expense  of  any  kind  to 
the  city. 

Expansion  Joints.  The  contractor  for  the  work  above 
specified  shall  also  be  required  to  provide  for  expansion 
joints  across  the  pavement  at  such  locations  as  may  be 
necessary,  which  expansion  joints  shall  extend  through 
the  blocking  and  concrete  and  shall  be  filled  with  a  com- 
position especially  prepared  for  the  purpose  according 
to  the  Blome  Company  patents.  These  expansion  joints 
shall  be  constructed  in  an  extremely  careful  manner  under 
specific  direction  of  the  city  engineer. 

For  the  purpose  of  diminishing  the  wear  at  the  expan- 
sion joints,  a  method  of  reinforcement  has  been  devised 
and  patented,  which  protects  the  corners  of  concrete 
at  the  joints.  This  protection  may  be  applied  to  any 
concrete  surface.  A  pavement  is  constructed  by  the 
owners  of  this  patent,  known  as  the  Baker  Armored 
Concrete  Pavement,  specifications  for  surface  and  joints 
of  which  are  as  follows : 


WEARING  SURFACE. 

"  After  placing  the  above  concrete  base,  and  before 
it  has  taken  its  initial  set,  there  shall  be  placed  thereon 
a  two  (2)  inch  wearing  surface  of  the  following  component 
parts : 

"One  (i)  part  of  cement  to  one  and  one-half  (i  J)  parts 
of  clean,  sharp  sand,  and  three  (3)  parts  of  hard  head 
pebbles  of  a  uniform  size  to  be  not  less  than  one-quarter 
(-J-)  inch  in  diameter,  and  not  more  than  one-half  (J) 


CONCRETE  PAVEMENTS.  353 

inch  in  diameter.  The  proportion  of  sand  to  stone  given 
is  approximate  as  absorption  tests  may  show  the  necessity 
of  variation.  Stone  and  sand  to  be  screened  and  subjected 
to  tests  as  described  under  paragraph  No.  6  from  time 
to  time  as  the  work  progresses  in  order  to  insure  a  solid 
stone  with  all  the  voids  completely  filled  with  sand  and 
cement. 

ARMORED  EXPANSION  JOINT. 

"  Expansion  joints  one-half  (J)  inch  wide  shall  be  made 
wherever  necessary  to  provide  for  expansion  and  con- 
traction. Expansion  joints  to  be  filled  with  No.  6  pitch 
or  other  approved  material.  At  intervals  of  twenty-five 
(25)  feet,  expansion  joints  will  be  provided  extending 
from  curb  to  curb. 

"  Where  pavement  comes  in  contact  with  street  car 
or  other  tracks,  expansion  joints  shall  be  made  at  the  end 
of  ties  to  provide  for  vibration  caused  by  the  jar  of  pass- 
ing cars. 

"  All  expansion  joints  are  to  be  armored  and  sharp 
edges  protected  against  abrasion  by  means  of  angles  of 
J-inch  steel  plates,  3  inches  wide,  provided  with  shear 
members  which  tie  them  securely  to  concrete  base  and 
wearing  surface.  These  are  clamped  to  a  dividing  board 
shaped  to  conform  to  the  crown  of  the  street.  After 
pavement  has  been  finished,  the  dividing  board  must  be 
removed,  and  opening  covered  with  tar  paper  until 
filled  with  No.  6  paving  pitch  or  other  specified  material. 

"  After  the  pavement  has  been  finished  for  twenty- 
four  (24)  hours,  it  shall  be  covered  with  sand  or  shavings 
for  a  period  of  seven  (7)  days." 


354      A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  91.    MONOLITHIC   CONCRETE   PAVEMENTS. 

The  construction  of  pavements  of  a  single  layer  of 
concrete  without  the  mortar  surface  seems  to  be  to  some 
extent  superseding  the  mortar  surface  pavement,  and  to 
have  given  good  results  in  a  number  of  instances.  It  is 
claimed  for  this  construction  that  the  surface  is  not  so 
apt  to  wear  slippery,  and  that  the  larger  aggregate  offers 
greater  resistance  to  wear  than  the  mortar  made  of  fine 
materials.  The  comparative  value  of  the  two  methods  in 
any  particular  case  probably  depends  mainly  upon  the 
availability  of  suitable  materials,  and  the  cost  of  con- 
struction. The  aggregate  used  in  making  concrete  for 
this  purpose  should  be  hard,  tough  materials,  capable 
of  resisting  abrasion.  Hard  limestones,  granites,  or 
trap-rock  may  be  used,  and  the  concrete  must  be  very 
thoroughly  and  uniformly  mixed,  and  must  be  placed 
and  compacted  before  beginning  to  set.  The  propor- 
tions of  the  materials  in  the  concrete  should  be  such 
that  all  voids  are  thoroughly  filled.  A  mixture  of  two 
grades  of  rock  of  different  wearing  qualities  has  some- 
times been  used  for  the  purpose.  A  pavement,  in  which 
a  mixture  of  limestone  and  granite  is  used,  is  patented, 
and  known  as  the  Moran  Pavement. 

In  finishing  the  surface  of  a  pavement  of  this  type, 
the  concrete  is  tamped  until  the  mortar  flushes  to  the 
surface,  which  may  then  be  broomed  to  give  a  finish,  or 
marked  by  tamping  lightly  upon  planks  laid  transversely 
upon  it.  These  surfaces  are  not  commonly  grooved, 
but  expansion  joints  are  used,  as  with  the  mortar-surfaced 
pavements.  It  is  usually,  the  effect  of  contraction  rather 
than  that  of  expansion,  which  needs  to  be  provided  for 
in  such  work,  and  this  contraction  may  be  to  some  extent 
obviated  by  covering  the  surface  with  sand  and  keeping 


CONCRETE  PAVEMENTS.  355 

it  moist  for  a  considerable  time  after  placing  the  pave- 
ment. 

The  Association  for  Standardizing  Paving  Specifica- 
tions have  suggested  the  following  method  for  placing  a 
concrete  pavement: 

PLACING  CONCRETE. 

"  The  concrete  shall  be  deposited  in  a  layer  on  the 
sub-grade  in  such  quantities  that,  after  being  thoroughly 
rammed  in  place,  it  will  be  of  the  required  thickness, 
and  the  upper  surface  shall  be  true  and  uniform. 

"  In  conveying  the  concrete  from  the  place  of  mixing 
to  the  place  of  deposit,  the  operation  must  be  conducted 
in  such  a  manner  that  no  mortar  will  be  lost  and  the 
concrete  must  be  so  handled  that  it  will  be  of  uniform 
composition  throughout,  showing  no  excess  or  lack  of 
mortar  in  any  place. 

THICKNESS. 

"The  thickness  of  the  pavement  shall  be  —  inches, 
with  its  upper  surface  on  the  finished  grade. 

"  The  minimum  thickness  for  concrete  pavement  shall 
be  5  J  inches. 

FINISHING. 

"  The  pavement  shall  be  finished  by  thorough  hand 
tamping,  until  the  mortar  flushes  freely  to  the  surface, 
then  lightly  tamping  with  a  template  made  of  2-inch 
plank  shaped  to  conform  to  the  curvature  of  the  surface 
of  the  finished  pavement  and  having  a  length  of  not  less 
than  one-half  the  width  of  the  roadway  to  give  a  uniform 
surface  with  the  slight  markings  thus  made  transverse 
to  the  street. 


356     A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

EXPANSION  JOINTS. 

"  Expansion  joints  shall  be  placed  at  right  angles  to 
the  curb  line  at  intervals  of  50  feet.  These  joints  shall 
be  not  less  than  i  inch  wide  and  shall  be  filled  with 
creosoted  soft  wood  timber  with  the  grain  vertical  and 
extending  full  depth  of  the  pavement. 

PROTECTION  TO  WORK. 

"  During  the  first  four  days  after  placing,  the  pavement 
shall  be  kept  moist  and  it  shall  be  protected  against 
traffic  until  the  concrete  has  thoroughly  set.  In  no  event 
shall  the  pavement  be  used  within  ten  days  after  being 
laid." 

ART.  92.  GROUTED  CONCRETE  PAVEMENT. 

Concrete  pavements  are  sometimes  constructed  by 
placing  a  layer  of  broken  stone  upon  the  road-bed,  roll- 
ing to  a  firm  surface,  and  grouting  with  Portland  cement 
grout,  so  as  to  fill  the  voids  in  the  stone. 

A  pavement  of  this  kind  is  patented  and  known  as  the 
Hassam  Pavement.  In  the  construction  of  these  pave- 
ments, a  grouting  of  i  to  2  Portland  cement  mortar  is 
used  after  the  stone  has  been  firmly  compacted  by  rolling 
and  the  voids  reduced  to  a  minimum.  The  rolling  is 
continuous  during  the  process  of  grouting.  Upon  the 
surface  so  obtained,  a  thin  layer  of  pea  stone  is  spread, 
grouted  and  rolled  until  the  grout  flushes  to  the  surface. 

In  constructing  the  Long  Island  Motor  Parkway,  a 
Hassam  pavement  was  used,  which  was  reinforced  with 
wire  fabric.  In  this  case,  a  2  J- inch  layer  of  stone  was 
placed,  the  wire  fabric  laid  upon  this,  and  another  layer 
of  stone  of  the  same  depth  added.  The  stone  was  then 
rolled  and  the  pavement  finished  as  usual,  no  expansion 
joints  being  used. 


CHAPTER  XIII. 

CITY  STREETS. 
ART.  93.     ARRANGEMENT  OF  CITY  STREETS. 

THE  location  of  streets  should  be  planned  with  a 
view  to  giving  direct  and  easy  communication  between 
all  parts  of  a  city.  The  arrangement  should  also  be 
such  as  to  permit  the  subdivision  of  the  area  traversed 
by  them  in  such  a  manner  as  to  give  the  maximum  of 
efficiency  for  business  or  residential  purposes.  The 
most  obvious  and  satisfactory  method  of  accomplishing 
these  purposes  is  usually  by  the  use  of  the  rectangular 
system,  with  occasional  diagonal  streets  along  lines 
likely  to  be  in  the  direction  of  considerable  travel. 

Streets  so  far  as  possible  should  be  systematically 
arranged  and  continuous  throughout  the  extent  of  the 
city,  both  to  facilitate  travel  and  to  admit  of  their  being 
so  named  and  numbered  that  the  locality  of  a  place 
of  business  or  residence  may  at  once  be  evident,  from 
its  address,  to  any  one  familiar  with  the  general  plan 
of  the  city.  The  rectangular  system  is  desirable  on 
this  account,  and  also  because  it  furnishes  blocks  of  the 
best  form  for  subdivision  into  building  lots. 

The  proper  arrangement  of  streets  will  always  neces- 
sarily depend  in  some  measure  upon  the  natural  feat- 
ures of  the  locality,  and  any  system  of  arrangement 
will  be  more  or  less  modified  by  local  topography. 
Where  for  topographic  or  aesthetic  reasons  it  may  be 
considered  desirable  to  use  curved  lines  for  the  streets, 

357 


358       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  continuity  and  uniformity  of  arrangement  should 
be  maintained  as  far  as  possible.  The  use  of  curves 
on  residence  streets  may  sometimes  be  advantageous 
in  reducing  gradients  or  in  its  effect  upon  adjoining 
property  through  avoiding  heavy  earthwork.  Where 
a  change  in  direction  is  necessary  the  use  of  a  curve 
usually  gives  a  better  appearance  than  an  abrupt  bend, 
unless  the  change  can  be  effected  at  the  intersection  of 
a  cross-street.  Care  is  required,  however,  to  prevent 
the  local  introduction  of  curvature  disarranging  the 
general  plans  and  producing  the  chaotic  condition  due 
to  an  irregular  use  of  short  streets. 

In  laying  out  a  rectangular  system  of  streets  the 
blocks  ordinarily  will  preferably  be  long  and  narrow. 
The  distance  needed  between  streets  in  one  direction 
is  only  that  necessary  to  the  proper  depth  of  lots,  while 
in  the  other  direction  the  streets  need  only  be  close 
enough  to  provide  convenient  communication  for  the 
travel  and  traffic.  A  convenient  method  would  be  to 
lay  out  the  main  streets  so  as  to  form  squares  large 
enough  to  permit  the  introduction  of  an  intermediate 
minor  street  through  the  blocks.  These  minor  streets 
may  then  be  introduced  in  the  direction  that  seems 
advisable  in  each  locality.  Such  an  arrangement  is 
shown  in  Fig.  28.  The  diagonal  streets  cut  more  space 
from  the  blocks  traversed  by  them,  but  give  more 
frontage  and  property  fronting  them  will  usually  have 
more  value  than  other  property  in  its  vicinity. 

The  proper  location  for  diagonal  streets  intended  as 
thoroughfares  for  traffic  is  naturally  determined  by  the 
positions  of  the  business  centers  or  public  buildings 
and  parks,  from  which  they  may  radiate  in  such  manner 
as  to  bring  the  outlying  portions  of  the  city  into  the 
most  direct  communication  possible. 


CITY  STREETS. 


359 


A  city  cannot  usually  be  laid  out  complete.  Its  for- 
mation is  a  matter  of  gradual  growth  and  enlargement, 
and  the  end  cannot  be  seen  from  the  beginning.  For 
this  reason  it  is  frequently  necessary  to  undergo  great 
expense  in  the  larger  cities  in  cutting  new  streets  or  in 
changing  the  positions  or  dimensions  of  existing  old 
ones  in  built-up  districts  in  order  to  relieve  the 
crowded  condition  of  the  streets,  which  hampers  busi- 
ness and  renders  travel  difficult  and  unpleasant.  Much 
of  this  difficulty  might  frequently  be  obviated  if  in 


FIG.  28. 

growing  towns  and  cities  proper  attention  were  given 
to  the  regulation  of  suburban  development.  Such 
development  should  be  under  municipal  control  so 
far  as  to  require  at  least  that  each  new  subdivision 
which  opens  new  streets  should  be  made  with  a  view 
to  affording  proper  ways  of  communication  between 
adjoining  properties  by  making  streets  continuous. 
Where  such  regulation  does  not  exist  streets  will  be 
laid  in  any  manner  to  best  develop  the  particular  prop- 
erty in  which  they  are  placed. 

A  good  example  of  the  advantages  of  systematic  and 
liberal  plans  in  street  arrangement,  as  well  as  of  the 


360       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 


FIG.  29. 


CITY  STREETS. 


36l 


evils  of  unregulated  extension,  is  given  by  the  case  of 
Washington,  D.  C. 

Fig.  29  shows  a  portion  of  the  city  of  Washington 
illustrating  its  systematic  arrangement.  It  consists  of 
a  rectangular  system,  together  with  two  sets  of  diag- 
onal avenues,  and  open  squares  or  circles  at  the  inter- 
sections of  the  avenues. 

Fig.  30  shows  a  number  of  suburban  subdivisions  on 
the  borders  of  the  city  of  Washington,  made  previous 


nnDDQaD 


FIG.  30. 

to  the  adoption  of  the  law  regulating  them.  In  some 
cases  the  streets  of  adjoining  subdivisions  have  no 
communication  with  each  other,  and  the  general  ten- 
dency is  toward  a  labyrinth  of  short  streets.  The  law 
now  requires  that  all  street  extension  within  the  Dis- 
trict of  Columbia  shall  conform  to  the  general  plan  of 
the  city  of  Washington;  and  under  the  operation  of 
this  law  the  lines  of  many  of  the  city  streets  have  been 
extended  to  all  parts  of  the  District,  and  all  of  the 
suburban  development  is  being  gradually  brought  with 


362       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  city  into  one  harmonious  whole,  on  the  same  gen- 
erous plan  that  exists  within  the  city.  The  rectification 
of  the  irregular  plats  upon  the  borders  of  the  city  must,, 
however,  be  a  matter  of  heavy  expense  to  the  District. 

ART.  94.     WIDTH  AND  CROSS-SECTION. 

The  width  of  city  streets  is  important  both  on 
account  of  its  influence  upon  the  ease  with  which 
traffic  may  be  conducted,  and  because  of  its  effect  upon 
the  health  and  comfort  of  the  people,  by  determining 
the  amount  of  light  and  air  which  may  penetrate  into 
thickly  built-up  districts. 

To  properly  accommodate  the  traffic  of  commercial 
thoroughfares  in  business  districts  of  towns  of  consider- 
able size,  it  is  desirable  that  a  street  should  have  a 
width  of  100  to  1 60  feet,  the  whole  of  it  to  be  used  for 
roadway  and  sidewalks.  Wide  streets  are  especially 
needed  where,  as  in  the  larger  cities,  they  are  bordered 
by  high  buildings  or  are  to  carry  lines  of  street  railway. 

Residence  streets  in  a  town  of  considerable  size, 
where  houses  are  set  out  to  the  property  line  and  stand 
close  together,  should  have  a  width  of  at  least  80  to 
100  feet  in  order  to  look  well  and  give  plenty  of  light 
and  air. 

The  streets  in  nearly  all  large  towns  are  laid  out  too 
narrow;  they  are  crowded  and  dingy.  The  chief  diffi- 
culty is  that  the  future  of  a  street  is  not  usually  fore- 
seen when  it  is  located.  Owners  in  subdividing  prop- 
erty are  only  anxious  to  get  as  many  lots  as  possible 
out  of  it,  and  there  are  usually  no  regulations  looking 
to  the  future  health  and  comfort  of  residents  when  the 
street  shall  be  built  upon.  In  the  growth  of  a  town  the 
nature  of  localities  changes;  residence  streets  become 


CITY   STREETS.  363 

business  streets,  streets  devoted  to  retail  trade  become 
wholesale  streets,  and  mercantile  districts  are  given  up 
to  manufacturing.  If  a  city  could  be  laid  out  com- 
plete from  the  beginning  it  would  be  comparatively 
easy  to  consider  the  requirements  to  be  met  and  locate 
the  streets  accordingly.  Under  existing  conditions  this 
is  not  possible,  but  a  more  liberal  policy  in  planning 
streets  wrould  usually  be  found  of  advantage  in  any 
growth  that  may  ensue.  There  is  also  very  frequently 
an  immediate  financial  advantage  in  the  enhancement 
of  values  due  to  wide  streets.  A  lot  100  feet  deep  on 
a  street  80  feet  wide  will  nearly  always  be  of  greater 
value  than  if  the  same  lot  be  no  feet  deep  and  the 
street  only  60  feet  in  width. 

In  Washington,  D.  C.,  which  probably  has  the  best 
general  system  of  any  American  city,  no  new  street  can 
be  located  less  than  90  feet  in  width,  and  avenues 
must  be  at  least  120  feet  wide.  Intermediate  streets, 
called  places,  60  feet  wide,  are  allowed  within  blocks, 
but  full-width  streets  must  be  located  not  more  than 
600  feet  apart.  The  value  of  this  liberal  policy  to  the 
city  of  Washington  is  evident  not  only  in  the  increased 
comfort  of  the  people,  but  in  its  large  growth  as  a 
residential  city  and  the  increased  value  of  property 
in  it. 

While  it  is  advantageous  to  have  the  street  wide  be- 
tween building-lines,  it  is  not  necessary  that  the  whole 
street  width  be  used  for  pavements.  The  street  pave- 
ment should  be  gauged  in  width  by  the  immediate 
necessities  of  the  traffic  which  is  to  pass  over  it.  The 
pavement  should  be  wide  enough  to  easily  accommo- 
date the  traffic,  but  any  unnecessary  width  is  a  tax 
upon  the  community  in  the  construction  and  mainte- 
nance of  more  pavement  than  should  be  required,  and 


364        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

perhaps  diminishes  the  length  of  street  which  may  be 
improved  with  available  funds.  Thus,  for  a  residence 
street  in  general  a  width  of  30  to  35  feet  between 
curbs  is  usually  ample,  with  a  foot-walk  upon  each  side 
5  to  10  feet  wide.  The  remainder  of  the  street  width 
should  be  made  into  lawns  upon  each  side,  with  tree 
spaces  between  the  sidewalk  and  roadway. 

Fig.  31  shows  in  partial  section  the  arrangement  of  a 
90-foot  residence  street  for  moderate  traffic.  For  resi- 
dence streets  of  lesser  importance,  where  the  travel 
is  light  and  the  street  is  only  required  to  furnish 
facilities  to  meet  the  needs  of  its  immediate  locality,  a 
less  width  of  pavement  may  often  be  advantageously 


Lt.  -    -    -   ....  33  .Fr.      •*"-  3  ^  -  -8  R.:  »<  -     LAWN    17  Fh    -  • 

FIG.  31. 

used.  A  pavement  24  feet  wide  is  sufficient  to  accom- 
modate a  very  considerable  amount  of  light  driving, 
and  in  many  places,  especially  in  the  smaller  towns 
where  funds  for  effective  improvement  are  obtained 
with  difficulty,  even  less  widths  may  be  employed  with 
the  result  of  improving  the  streets  both  in  appearance 
and  usefulness.  All  that  is  really  needed  in  such  cases 
is  room  for  teams  to  pass  comfortably  and  to  turn 
without  difficulty.  The  narrowing  of  roadways  on 
streets  of  light  traffic  to  what  is  really  necessary  may 
often  make  possible  improvements  which  will  turn  a 
broad  sea  of  mud  into  a  narrow,  hard  roadway  and  a 
grass-plat.  Fig.  32  shows  the  arrangement  of  a  village 
street  50  feet  wide  for  light  service. 

In  many  cases  for  village  streets,  where  the  traffic  is 
light  and  it  is  essential  that  the  cost  of  construction  be 


CITY   STREETS.  365 

low,  it  may  be  good  practice  to  construct  the  traveled 
portion  of  the  roadway  of  macadam,  or  other  pave- 
ment, and  use  cobble  gutters  at  the  sides  without 


FIG.  32.   , 

curbs.  Fig.  33  shows  a  roadway  30  feet  wide,  with 
macadam  middle  and  cobble  gutters.  In  Saginaw, 
Mich.,  this  method  has  been  followed,  using  either 
macadam  or  wood  blocks  for  the  middle  portion,  and 
in  the  report  of  City  Engineer  Roberts  for  1893  it  is 
recommended  as  economical  and  efficient. 

The  cross-section  of  streets  must  be  arranged  with 
reference   to   proper   surface   drainage.     The   street   is 


FIG.  33. 

given  a  crown  at  the  middle  to  throw  the  water  into 
the  gutters,  and  sidewalks  usually  have  a  sufficient  in- 
clination toward  the  gutter  to  cause  them  to  drain  over 
the  curb.  The  crown  necessary  to  insure  good  drain- 
age in  the  roadway  depends  upon  the  nature  of  the 
covering,  being  less  as  the  surface  is  more  smooth  and 
less  permeable  to  water.  For  macadam  roadways,  it 
may  vary  from  about  ?V  to  ffV  °f  the  width  of  the 
roadway.  For  the  various  pavements,  the  required 
crown  varies  from  about  gV  to  T^o  of  the  width,  accord- 
ing to  the  smoothness  of  the  surface  and  the  permea- 
bility of  the  construction.  For  brick,  asphalt,  or 
wood-block  surfaces,  a  crown  of  from  -fa  to  TJff  of 


366       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  width  is  commonly  ample.  Stone  blocks  may 
need  slightly  more;  while,  on  streets  of  considerable 
longitudinal  slope,  the  crown  may  be  made  somewhat 
lighter. 

The  form  of  section  is  usually  a  convex  curve, 
sometimes  circular  but  more  often  parabolic,  the  para- 
bolic curve  differing  but  slightly  from  the  circular. 
This  form  is  shown  in  Fig.  34.  The  distance  of  the 


FIG.  34. 

curved  surface  below  the  horizontal  through  the  highest 
point  is  proportional  to  the  square  of  the  horizontal 
distance  from  the  center.  Thus,  if  the  distance  A  —  D 
be  divided  into  3  equal  parts,  the  vertical  distance  from 
B  to  the  curve  is  one-ninth  and  from  C  four-ninths  of 
that  at  D. 

The  street  is  usually  made  practically  level  across, 
the  curbs  and  sidewalks  at  the  two  sides  being  given  the 


FIG.  35. 

same  elevation.  The  parking  at  the  sides  may  have  a 
slope  between  the  sidewalk  and  the  building-line  when 
it  is  necessary  or  advantageous.  Sometimes,  on  streets 
along  a  slope,  expense  may  be  saved  or  adjoining 
property  benefited  by  placing  the  sidewalk  at  a  dif- 
ferent elevation  from  that  of  the  street,  as  shown  in  Fig. 
5,  or  by  placing  one  curb  lower  than  the  other  and 
moving  the  crown  of  the  road  to  one  side,  as  shown  in 
Fig.  35- 


CITY   STREETS.  '367 

The  surface  drainage  of  alleys  is  secured  either  by 
forming  the  section  as  in  a  street,  with  a  crown  at  the 
middle  and  gutters  and  curbs  at  the  sides,  or,  as  is  com- 
monly preferable  with  narrow  alleys,  by  placing  the 
gutter  at  the  middle  and  sloping  the  pavement  from 
the  sides  to  the  center.  Where  the  gutter  is  in  the 
middle  it  is  common  to  make  the  bottom  of  the  gutter 
of  a  flagstone  15  to  1 8  inches  wide.  Fig.  36  shows  a 


FIG.  36. 

center-drained  alley  with  block-stone  pavement  upon 
sand  foundation. 

The  form  shown  in  Fig.  36  is  also  usually  emplo3Ted 
where  concrete  pavement  is  used  for  alleys,  as  is  quite 
common.  This  is  desirable  in  many  instances  on  ac- 
count of  the  good  drainage  afforded,  and  the  resistance 
of  the  material  to  dampness. 

Fig.  37  shows  a  side-drained  cobble  pavement  for  an 

FIG.  37. 

alley.  These  have  been  extensively  used  in  the  past, 
being  usually  placed  upon  sand  foundation.  They  are 
gradually  being  replaced  by  brick  or  concrete  pave- 
ments. 

ART.  95.     STREET  GRADES. 

The  grades  of  city  streets  necessarily  depend  mainly 
upon  the  topography  of  the  site.  Wherever  possible, 


368       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

it  is  desirable  that  grades  be  uniform  between  cross- 
streets. 

In  establishing  grades  for  new  streets  through  unim- 
proved property,  they  may  usually  be  laid  with  refer- 
ence only  to  obtaining  the  most  desirable  gradients  for 
the  street  within  a  proper  limit  of  cost.  But  where 
improvements  have  already  been  made,  and  located 
with  reference  to  the  natural  surface  of  the  ground,  it 
is  frequently  a  matter  of  extreme  difficulty  to  give  a 
desirable  grade  to  the  streets  without  injury  to  adjoin- 
ing properties.  In  such  cases  it  becomes  a  question  of 
how  far  individual  interests  shall  be  sacrificed  to  the 
general  good.  It  may  be  said  in  this  connection  that 
adjustments  to  new  grades  are  usually  accomplished 
much  more  easily  than  would  be  anticipated,  and  when 
accomplished  the  possession  of  a  desirable  grade  is 
of  very  considerable  value  to  adjoining  property.  Too 
great  timidity  should  not,  therefore,  be  felt  in  regard 
to  making  necessary  changes  because  of  the  fear  of  in- 
juring property  in  the  locality. 

Where  a  grade  if  made  continuous  between  inter- 
secting streets  would  be  nearly  level,  it  is  frequently 
necessary  to  put  a  summit  in  the  middle  of  the  block 
and  give  a  light  gradient  downward  in  each  direction 
to  the  cross  streets  in  order  to  provide  for  surface  drain- 
age. The  amount  of  slope  necessary  to  provide  for 
proper  drainage  depends  upon  the  character  of  the  sur- 
face and  smoothness  of  the  gutter.  For  a  surface  of  earth 
or  macadam  the  slope  should  not  be  less  than  about  I 
in  100,  and  for  paved  streets  from  I  in  200  to  I  in  250. 

In  some  cases  it  may  be  possible  to  give  sufficient 
slope  to  gutters  to  carry  off  the  surface-water  by  mak- 
ing the  gutter  deeper  at  the  ends  than  in  the  middle  of 
the  block  without  making  a  summit  in  the  crown  of 


CITY  STREETS.  369 

the  street.  The  curb  in  such  case  would  be  made 
level  or  of  uniform  gradient. 

It  may  frequently  be  necessary  to  consider  the 
effect  of  grade  in  determining  the  character  of  pave- 
ment to  be  employed  upon  a  street.  Asphalt  is  com- 
monly limited  to. grades  of  4  or  5  per  cent,  although 
some  engineers  use  it  on  6  or  7  per  cent  grades.  Brick 
is  commonly  used  on  grades  up  to  about  8  per  cent, 
and  in  some  places  has  given  satisfactory  service  on  10 
per  cent  grades.  Wide  joints,  about  £  inch,  are  some- 
times used  in  brick  pavements  on  steep  streets,  in  order 
to  afford  a  better  foothold  for  horses.  This,  however, 
in  other  instances  appears  to  be  unnecessary,  provided 
the  pavement  is  kept  clean  and  in  good  condition. 

Wood  blocks  may  safely  be  used  on  grades  of  5  or 
6  per  cent,  while  smooth  stone  blocks  are  employed  in 
about  the  same  manner  as  bricks,  being  if  anything  a 
little  more  slipper}7  than  bricks.  Stone  blocks  of  some- 
what rough  character  are  successfully  used  in  some 
instances  on  grades  of  12  or  13  per  cent. 

In  a  report  on  the  streets  of  Duluth  in  1890,  Messrs. 
Rudolph  Hering  and  Andrew  Rosewater  recommend 
for  steep  streets,  in  addition  to  the  above,  that  brick 
may  be  used  in  which  the  tops  are  rounded,  and  that 
wood  blocks  for  such  use  have  their  upper  edges  cham- 
fered on  each  side,  or  if  round  blocks  be  used,  around 
the  blocks.  Subsequent  experience  has,  however, 
seemed  to  indicate  that,  except  in  extreme  cases,  such 
special  construction  is  not  necessary. 

On  the  streets  too  steep  for  smooth  pavement  it  is  not 
unusual  to  pave  part  of  the  street  width  with  a  smooth 
pavement,  like  asphalt,  and  the  remainder  with  stone 
blocks  or  some  rough  pavement  for  use  in  slippery 
weather. 


370       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  96.     STREET  INTERSECTIONS. 

At  intersections  the  crown  of  the  roadway  pavement 
on  each  street  should,  if  possible,  be  continuous  to  the 
center  of  intersection,  in  order  to  prevent  vehicles  on 
one  street  from  being  subjected  to  the  jar  incident  to 
passing  over  the  gutter  of  the  other.  Where  a  storm- 
sewer  is  available  into  wrhich  the  water  from  the  gut- 
ters on  the  upper  side  can  be  emptied  this  is  a  simple 
matter,  but  where  such  sewers  do  not  exist  it  requires 
the  adoption  of  some  special  means  of  draining  the 
gutters  on  the  upper  side.  This  may  sometimes  be 
accomplished  by  a  culvert  across  the  street,  the  gutters 
being  somewhat  depressed  at  the  corners  to  bring  the 
channel  sufficiently  low.  In  other  cases,  where  the 
slope  is  sufficient,  it  is  more  satisfactory  to  construct 
an  underground  pipe-drain  from  the  upper  corner  to 
some  point  in  the  gutter  below  the  crossing. 

Where  the  rate  of  grade  is  such  that  it  is  feasible,  it 
is  desirable  that  the  grade  of  both  streets  should  be 
brought  to  a  level  at  intersections.  The  top  of  the  curb 
at  the  four  corners  should  be  at  the  same  elevation,  thus 
permitting  the  continuation  of  the  full  section  of  each 
roadway  until  they  intersect.  It  is  also  desirable  that 
the  sidewalks  at  the  corners  be  level;  that  is,  the  points 
a  a  in  Fig.  38  should  all  be  placed  at  the  same  eleva- 
tion, which  will  make  the  entire  street  section,  includ- 
ing sidewalks,  horizontal  across  the  direction  of  travel 
on  each  street. 

On  very  steep  slopes  it  may  not  be  possible  to  flatten 
out  the  grade  to  a  level  in  crossing  transverse  streets, 
and  in  such  cases  the  elevations  require  study,  and  need 
to  be  carefully  worked  out  for  each  particular  case.  In 
the  report  of  Messrs.  Rudolph  Hering  and  Andrew 


CITY   STREETS. 


371 


Rosewater  upon  the  streets  of  Duluth,  it  is  recom- 
mended that  in  all  cases  the  grade  shall  be  reduced  to 
3  per  cent  between  the  curb  lines  of  cross  streets,  and 
the  grade  of  the  curb  reduced  in  all  cases  to  8  per  cent 
for  the  width  of  the  sidewalks  of  intersecting  streets. 
This  is  to  be  considered  the  maximum  allowable  rate 
of  transverse  grade,  and  only  to  be  employed  in  case  of 
necessity.  If  in  Fig.  38  the  arrow  represents  the  direc- 
tion of  steep  slope,  and  the  street  transverse  to  that 
direction  has  a  roadway  40  feet  wide  with  sidewalks 


a 


FIG.  38. 

10  feet  wide,  the  above  limits  would  permit  the  curb 
at  c  to  be  1.2  feet  lower  than  that  at  b,  and  admit  of 
a  fall  of  0.8  foot  in  the  curb  line  from  a  to  b  and  from 
c  to  d.  If  both  streets  have  the  same  grade  and 
width  the  curb  at  the  lowest  corner  would  be  2.4  feet 
lower  than  at  the  highest  corner. 

Sometimes,  where  the  parallel  streets  in  one  direction 
follow  the  lines  of  greatest  slope,  and  the  cross  streets 
are  normal  to  them,  the  proper  grades  at  intersections 
may  be  arranged  by  giving  the  streets  along  the  slope 
a  section  similar  to  that  shown  in  Fig.  32  throughout 
its  length,  thus  permitting  the  street  in  the  direction 


372       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

of  slope  to  continue  its  grade  across  the  intersection 
without  altering  at  that  point  the  side  slope  of  the 
cross  street. 

For  a  case  of  maximum  slope  this  would  make  the 
section  of  the  roadway  of  the  cross  street  a  plane  sur- 
face sloping  uniformly  from  the  upper  to  the  lower 
curb,  or  in  Fig.  35  it  would  transfer  the  street  crown  to 
the  upper  curb. 

ART.  97.     FOOTWAYS. 

Footways  are  not  required  to  bear  the  heavy  loads 
which  come  upon  the  roadway  pavement,  but  in  streets 
of  considerable  travel  are  subjected  to  a  continual 
abrading  action,  and  for  good  service  are  required  to 
be  of  a  material  which  will  resist  abrasion  well,  of  so 
uniform  a  texture  as  to  wear  evenly,  and  not  hard 
enough  to  become  smooth  and  slippery  in  use. 

A  good  sidewalk  should  always  present  an  even  sur- 
face, and  therefore  requires  a  firm  foundation  to  resist 
the  displacement  of  the  blocks  of  which  it  may  be  com- 
posed. It  must  also  be  durable  under  atmospheric 
changes,  and  of  material  that  may  be  easily  cleaned. 
The  materials  commonly  employed  are  gravel,  wood, 
brick,  asphalt,  stone,  and  concrete. 

Gravel  walks  are  the  cheapest  of  footways  where 
suitable  material  is  available.  They  are  constructed  in 
a  manner  similar  to  that  used  for  gravel  roadways,  and 
require  that  the  bed  of  the  walk  be  well  drained,  and 
that  it  be  well  compacted  by  rolling  or  ramming  before 
the  walk  is  placed  upon  it.  The  best  gravel  walks  are 
usually  built  upon  a  base  of  rough  stone.  This  base 
may  be  6  or  8  inches  thick,  and  forms  a  solid  founda 
tion  upon  which  the  gra  vel  surface  may  be  placed  and 


CITY   STREETS.  373 

sustained  against  settling.  Walks  constructed  in  this 
manner  are  frequently  used  in  city  parks  where  the 
travel  is  considerable.  On  suburban  roads  gravel 
walks  usually  consist  of  a  thin  surface  of  gravel  laid 
upon  the  earth-bed,  and  are  replaced  by  some  other 
surface  when  a  more  expensive  construction  can  be 
afforded.  Gutters  are  frequently  necessary  to  protect 
the  walk  from  the  wash  of  surface-water,  which  other- 
wise very  quickly  destroys  it. 

Wood  is  commonly  used  for  walks  in  the  form  of 
planks  which  are  laid  on  stringers,  the  planks  being 
placed  perpendicularly  to  the  direction  of  travel.  It  is 
comparatively  short-lived,  and  requires  considerable 
expenditure  for  repairs,  as  the  material  is  perishable 
and  also  wears  rapidly. 

Brick  footway  pavements  have  been  extensively  used 
for  many  years,  and  form,  when  well  constructed,  a  very 
durable  and  satisfactory  sidewalk.  As  commonly  con- 
structed they  consist  of  ordinary  hard-burned  bricks 
laid  flat  upon  a  layer  of  sand  over  the  earth-bed.  For 
light  travel,  pavements  so  constructed  may  last  well 
and  give  good  service;  but  they  are  apt  to  soon  become 
uneven  through  the  sinking  of  the  bricks  because  of  in- 
sufficient foundation. 

In  constructing  such  a  pavement  the  sand  layer 
should  be  well  compacted  by  rolling  or  ramming  be- 
fore setting  the  bricks,  which  should  also  be  rammed 
to  a  firm  and  even  bearing.  To  give  satisfactory  re- 
sults a  foundation  of  sand  and  gravel  or  broken  stone 
should  be  formed  8  to  10  inches  in  thickness.  In 
Washington  a  layer  of  gravel  4  inches  thick  and  well 
compacted  is  used,  with  a  layer  of  sand  of  the  same 
thickness  upon  it  to  receive  the  surface.  In  forming 
the  pavements,  the  bricks  are  laid  flat  and  as  close  as 


374        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

possible.  The  joints  are  filled  with  sand,  usually  by 
coating  the  surface  with  a  layer  of  sand  before  ram- 
ming and  after  completion  a  second  coating,  which  is 
allowed  to  remain  a  few  days  after  admitting  the 
travel  to  it. 

Care  must  be  used  in  selecting  brick  for  this  purpose 
to  get  only  hard-burned  brick  of  uniform  qualit}^ 
in  order  that  the  resistance  to  wear  may  be  even. 
The  use  of  vitrified  paving  brick,  as  used  for  roadway 
pavement,  would  be  of  advantage  on  walks  subjected 
to  lieavy  wear. 

The  use  of  a  concrete  foundation  and  setting  the 
brick  on  edge  and  in  mortar,  after  the  manner  of  con- 
structing a  roadway  pavement,  makes  a  very  durable 
sidewalk  under  heavy  travel.  It  is,  however,  some- 
what expensive,  and  usually  a  stone  surface  would  be 
preferable  where  such  expense  is  to  be  incurred. 

Footway  pavements  of  a  concrete  in  which  coal-tar 
is  the  binding  material  have  been  widely  used,  but  have 
not  usually  been  satisfactory  in  use.  As  commonly 
constructed  they  wear  rapidly  and  soften,  becoming 
very  disagreeable  in  hot  weather.  Some  pavements 
of  this  character  have,  however,  shown  fairly  good 
service. 

Numerous  methods  have  been  proposed  and  tried 
for  the  construction  of  tar  footwalks,  differing  from 
each  other  in  the  materials  mixed  with  the  tar  to  form 
the  concrete,  and  in  the  manipulation  of  the  process. 
Ashes  mixed  with  sand  and  gravel  are  usually  em- 
ployed, and  sometimes  clinkers  from  an  iron  foundry  t 
A  somewhat  successful  pavement  of  this  class  has  a 
small  amount  of  Portland  cement  mixed  with  the  ashes 
and  sand  used  in  forming  the  concrete  before  the  addi- 
tion of  the  tar. 


CITY   STREETS,  375 

Asphalt  footway  pavements  are  formed  either  of  as- 
phalt blocks  or  of  a  surface  of  sheet  asphalt.  Where 
blocks  are  used  they  are  laid  in  the  same  manner  as 
brick  upon  a  foundation  of  sand  or  gravel.  The 
blocks,  or  tiles  as  they  are  commonly  called,  are  usu- 
ally made  flat,  about  8  inches  square  and  2  to  2j  inches 
thick.  They  are  laid  with  their  edges  either  at  right 
angles  to  the  street  line  or  at  an  angle  of  45°  with 
the  street  line  —  usually  at  right  angles,  on  account 
of  greater  ease  in  laying. 

Sheet-asphalt  footways  are  laid  in  the  same  manner 
as  an  asphalt  street  pavement,  the  pavement,  however, 
being  given  a  less  thickness.  In  Washington,  D.  C, 
these  pavements  are  made  about  3  inches  thick,  and 
constructed  upon  a  bituminous  base.  Material  re- 
moved from  street  pavements  in  re-surfacing  is  used 
for  forming  the  surface  material  of  the  footway. 

In  Europe  rock  asphalt  is  frequently  used  for  foot- 
ways, Asphalt  mastic  is  commonly  emplo\7ed,  mixed 
with  sand  or  gravel  to  give  a  wearing  surface.  The  in- 
gredients are  heated  together  and  applied  hot  to  a 
broken-stone  or  concrete  foundation.  In  Europe  hy- 
draulic cement  concrete  is  used  for  the  base,  as  in  the 
driveways.  A  la\Ter  of  3  or  4  inches  of  concrete  is  em- 
ployed, with  a  surface  layer  of  rock  asphalt  or  asphalt 
mastic  and  sand,  i  to  }  inch  in  thickness  for  ordinary 
work. 

Natural  stone  for  footwalks  is  ordinarily  used  in  the 
form  of  flagging.  Where  flagstones  of  proper  size  and 
good  wearing  qualities  may  be  readily  obtained,  this 
kind  of  pavement,  if  well  laid,  makes  a  durable  and 
satisfactory  footwalk.  Flagstones  should  be  set  upon 
a  solid  foundation  and  be  firmly  bedded  so  as  to 
preserve  an  even  surface.  They  should  not  be  laid,  as 


376       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

is  common  in  many  places,  directly  upon  an  earth-bed 
but  should  have  a  cushion  layer  of  sand  or  of  some 
porous  material  to  prevent  unequal  settling  under  the 
action  of  frost, 

CONCRETE   SIDEWALKS. 

Concrete  pavements,  when  well  constructed  of 
good  materials,  make  the  most  satisfactory  of  foot- 
ways. They  form  an  even  surface,  quite  agreeable  in 
service,  and  are  durable  and  economical  where  exposed 
to  considerable  travel. 

In  the  construction  of  a  concrete  sidewalk  a  base 
of  cinders  is  usually  employed,  supporting  a  layer  of 
rather  meager  concrete  and  a  thin  surface  layer  of 
cement  mortar.  The  cinders  are  commonly  4  to  8 
inches  thick,  6  inches  being  ample  for  most  walks,  and 
4  inches  being  sufficient  for  walks  in  residence  districts 
of  small  travel,  where  the  soil  is  firm.  Care  should  be 
taken  to  insure  the  proper  drainage  of  the  base,  so 
that  water  may  not  remain  in  the  soil  immediately 
under  the  walk,  or  stand  in  the  cinders.  The  cinders 
should  be  placed  to  proper  depth  and  well  camped 
with  the  upper  surface  parallel  to  the  finished  top  of 
the  pavement. 

The  concrete  base  is  usually  3  or  4  inches  thick,  and 
sometimes  on  streets  of  heavy  traffic  it  is  made  5  inches. 
The  wearing  coat  is  from  J  inch  to  I  inch  in  thickness, 
depending  upon  the  wear  to  which  it  is  to  be  subjected. 
A  concrete  base  3i  inches  thick,  with  a  wearing  sur- 
face J  inch  thick,  makes  a  very  satisfactory  walk  for 
residence  streets  carrying  moderate  travel. 

The  composition  of  the  concrete  base  must  depend 
largely  upon  the  materials  available  in  the  locality. 
Either  gravel  or  broken  stone  may  be  used,  with  or 


CITY  STREETS.  377 

without  sand,  according  to  the  character  of  the 
materials.  A  mixture  of  one  part  Portland  cement, 
three  parts  sand,  and  six  parts  broken  stone  is  com 
monly  used.  When  good  limestone  is  available,  a 
mixture  of  one  part  Portland  cement  to  four  parts 
broken  stone,  without  sand,  is  found  very  satisfactory, 
the  stone  being  broken  to  pass  a  one  inch  screen  and 
with  only  the  fine  dust  removed.  When  sand  or  gravel 
is  used,  it  is  important  that  it  be  clean,  as  any  dirt  is 
likely  to  work  to  the  surface  in  tamping  and  prevent  the 
proper  adhesion  of  the  surface  layer.  For  the  same 
reason,  the  concrete  must  not  be  mixed  too  wet,  and  it 
should  be  well  compacted  by  ramming. 

The  wearing  coat  is  composed  of  Portland  cement 
mortar,  one  part  cement  to  one  or  two  parts  sand  or 
screenings.  The  amount  of  cement  used  should  be 
sufficient  to  fill  the  voids  in  the  sand  but  not  greatly  in 
excess,  as  the  resistance  to  abrasion  is  lessened  by 
excess  of  cement.  The  material  for  wearing  coat  should 
be  either  clean,  hard  sand,  or  screenings  from  the 
crushed  stone.  The  screenings  should  have  the  very 
fine  dust  removed,  and  when  from  a  good  quality  of 
rock  are  superior  to  most  natural  sands.  The  mortar 
is  brought  to  a  uniform  surface  by  drawing  a  straight 
edge  along  the  tops  of  the  forms  at  the  sides  of  the  walk. 
The  surface  is  then  worked  smooth  and  uniform  with  a 
float  and  finished  with  a  plastering  trowel. 

Joints  should  be  left  at  intervals  of  4  or  5  feet  to 
prevent  irregular  cracks  through  contraction  of  the 
concrete.  These  joints  need  not  all  extend  through 
the  base  of  the  walk,  but  at  intervals  of  3  or  4  joints 
one  should  extend  through.  The  surface  of  a  concrete 
walk  should  have  a  transverse  slope  of  about  \  in&h 
to  I  foot  to  provide  for  proper  surface  drainage.  For 


378       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

residence  streets  of  moderate  travel  a  width  of  four  to 
six  feet  is  commonly  required.  Four  feet  is  incon- 
veniently narrow  unless  the  street  is  very  little  used, 
while  six  feet  is  sufficient  for  a  very  considerable 
amount  of  travel. 

ART.  98.     CURBS  AND  GUTTERS. 

Curbs  are  usually  set  in  the  streets  of  towns  at  the 
sides  of  roadway  pavements  for  the  purpose  of  sus- 
taining and  protecting  the  sidewalk  or  tree  space,  and 
of  forming  the  side  of  the  gutter.  They  are  commonly 
formed  of  natural  stone  or  concrete,  but  sometimes  clay 
blocks  are  used. 

STONE   CURBSo 

The  curbs  used  in  different  places  vary  considerably 
in  form  and  dimensions.  Stone  curbs  vary  from  4  to 
12  inches  in  width  and  from  8  to  24  inches  in  depth. 
They  are  usually  employed  from  3  to  6  feet  in  length 
and  set  with  close  joints. 

The  depth  must  be  sufficient  to  admit  of  their  being 
firmly  bedded,  and  to  prevent  overturning  into  the 
gutter.  The  front  of  the  curb  should  be  hammer- 
dressed  to  a  depth  greater  than  its  exposure  above  the 
gutter,  and  the  back  deep  enough  to  permit  the  side- 
walk pavement  to  fit  close  against  it  where  the  side- 
walk adjoins  the  curb.  The  ends  of  the  blocks  should 
also  be  dressed  to  the  depth  of  exposure,  and  the  part 
below  the  ground  trimmed  off  so  as  to  permit  the 
dressed  ends  to  come  in  contact  when  laid. 

Granite  is  usually  considered  the  best  material  for 
curbs,  although  both  sandstones  and  limestones  are 
used  in  many  places.  In  the  vicinity  of  New  York  the 


CITY   STREETS. 


379 


North  River  bluestone  has  proved  a  good  material  for 
the  purpose. 

There   are  various  ways  of  setting  the  curb.     The 
object  should  be  to  bed  it  firmly  on  a  solid  foundation. 


FIG.  39. 

The  best  method  is  to  place  a  bed  of  concrete  under  it. 
This  construction  is  shown  in  Fig.  39,  which  repre- 
sents the  method  used  in  setting  granite  curb  in  Wash- 


FIG.  40. 

ington,  D.  C.  The  curb  is  held  firmly  in  place  by  the 
concrete  foundation,  which  joins  it  rigidly  to  the  road- 
way pavement. 

Where  the  concrete  foundation  is  not  used  under 


380       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  curb  a  deeper  curbstone  is  necessary,  usually  from 
1 8  to  24  inches  in  good  work.  Curbs  are  very  com- 
monly set  in  the  natural  ground,  the  pavement  coming 
against  it  on  one  side;  but  it  is  usually  found  advan- 
tageous to  lay  them  upon  a  bed  of  gravel  or  broken 
stone,  with  gravel  filled  in  the  trench  about  them. 
The  ordinary  method  of  setting  curbs  is  shown  in 
Fig.  40. 

The  Washington  specifications  for  ordinary  work 
require  that  a  bed  of  gravel  4  inches  deep  be  used 
under  the  curb,  and  that  the  trench  be  filled  with 
gravel  placed  in  layers  3  or  4  inches  deep,  each  layer 
being  thoroughly  rammed  before  adding  the  next. 

CONCRETE  CURBS. 

Concrete  curbs  are  extensively  used,  and  their  use  is 
rapidly  increasing,  particularly  in  those  sections  where 
suitable  natural  stone  does  not  occur.  These  curbs 
consist  of  concrete  built  in  place  in  plank  forms, 
extending  continuously  along  the  street,  occasional 
joints  being  introduced  to  prevent  irregular  cracking. 
The  concrete  is  laid  and  surfaced  in  the  same  manner 
as  in  sidewalk  work,  all  exposed  faces  being  surfaced 
with  mortar.  For  curbs  upon  streets  of  light  travel 
concrete  mixed  in  the  same  proportions  as  for  sidewalks 
gives  good  service;  but  where  the  use  is  more  severe,  a 
richer  mixture  will  afford  more  strength,  and  about  one 
part  cement,  two  parts  sand,  and  four  of  broken  stone 
are  frequently  employed. 

The  curb  usually  extends  to  the  bottom  of  the  base 
of  the  pavement,  the  concrete  base  joining  the  curb  and 
holding  it  in  place.  For  residence  streets  the  curb  is 
commonly  5  or  6  inches  thick,  while  upon  heavy  traffic 


CITY   STREETS.  381 

streets  it  may  be  made  8  or  10  inches  thick.  Where 
the  traffic  is  very  severe,  concrete  curbs  are  frequently 
reinforced,  or  faced,  with  steel  at  the  edges  for  greater 
resistance  to  shocks.  Several  forms  of  patented  rein- 
forcement are  available  for  such  use. 


BURNED   CLAY   CURBS. 

Burned  clay  curbs  are  sometimes  employed  in  small 
towns  where  brick  pavements  are  used.  These  are 
made  in  such  small  sizes  and  short  lengths  that  they 
are  difficult  to  set  to  good  line  and  are  easily  displaced. 
They  have  not  in  general  proved  satisfactory. 

Curbs  at  street  corners  and  driveways  are  commonly 
laid  upon  curves.  On  ordinary  residence  streets  curves 
of  4  to  6  feet  radius  are  usually  employed,  while 
upon  wide  streets  with  considerable  traffic  curves  of 
8  to  12  feet  radius  are  desirable.  .  These  are  easily  set 
for  concrete  curbing  by  having  forms  to  fit  the  whole 
curve.  For  curves  of  3  or  4  feet  radius  stone  curbing 
may  be  cut  in  two  pieces  for  a  right-angled  turn. 
With  curves  of  larger  radius  more  pieces  must  be  used 
and  the  cutting  and  setting  becomes  more  expensive. 

GUTTERS. 

Gutters  are  commonly  formed  of  the  same  material 
as  the  roadway  pavement,  which  is  simply  extended 
to  the  curb. 

In  streets  paved  with  brick  or  granite  blocks  the 
gutter  blocks  are  sometimes  turned  lengthwise  of  the 
street,  as  shown  in  Fig.  22,  for  the  purpose  of  facilitat- 
ing the  flow  of  water  in  the  gutter.  As  already  pointed 
out,  however,  this  has  the  effect  of  making  a  continuous 


382        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS 

joint  between  the  pavement  and  gutter,  and  its  utility 
seems  doubtful. 

For  streets  paved  with  broken  stone  it  is  common  to 
employ  stone  gutters,  formed  of  cobblestones,  of  narrow 
flags  laid  lengthwise  of  the  gutter,  or  sometimes  of  rec- 
tangular blocks.  Such  construction  is  shown  in  Fig.  40. 
On  streets  paved  with  wood  these  gutters  may  also  be 
frequently  employed  with  advantage,  especially  where 
for  any  reason  the  gutter  is  likely  to  be  kept  damp. 
In  forming  a  cobble  gutter  the  stones  are  usually  set 
upon  a  layer  of  sand  or  gravel  after  the  manner  of 
forming  a  cobble  pavement.  They  should  be  firmly 
bedded  and  form  an  even  surface. 

Cobble  gutters  are  often  used  on  village  streets 
where  no  curbs  are  set,  and  in  such  locations,  where 
but  slight  expense  is  admissible,  they  are  quite  satis- 
factory if  properly  constructed.  This  method  of  con- 
struction is  illustrated  in  Fig.  33. 

Sometimes  in  work  of  this  kind  a  flagstone  is  used 
for  the  bottom  of  the  gutter  and  the  sides  are  formed  of 
cobble.  This  is  preferable  as  affording  a  more  free 
channel  for  the  flow  of  the  surface  drainage. 

To  obtain  satisfactory  results  it  is  always  necessary 
that  the  foundation  be  of  sufficient  depth  and  well 
compacted  in  order  to  prevent  the  surface  becoming 
uneven  by  the  stones  being  forced  downward  into  the 
road-bed  in  wet  weather  or  through  the  action  of  frost. 
A  layer  of  6  to  10  inches  of  gravel  or  sand  is  usually 
required. 

Where  flagstones  are  used  to  form  the  gutter,  they 
should  be  3  or  4  inches  thick,  10  to  15  inches  wide,  as 
may  be  required,  and  about  3  feet  long.  Care  is 
required  in  laying  that  they  may  have  an  even  bed  and 
be  well  supported  by  the  foundation. 


CITY   STREETS  383 

Gutters  of  bricks,  or  of  stone  blocks,  are  often  used  for 
streets  upon  which  the  roadway  pavement  is  asphalt, 
on  account  of  the  liability  of  the  asphalt  being  injured 
by  dampness.  In  this  case  the  gutter  is  constructed 
by  setting  the  bricks  or  blocks  with  their  greatest 
length  along  the  street.  They  are  placed  upon  a  bed 
of  concrete,  the  same  as  is  used  for  the  foundation 
under  the  asphalt  surface,  and  the  joints  are  filled 
with  hydraulic  cement  mortar,  as  in  constructing  brick 
pavement. 

CONCRETE    GUTTERS. 

Concrete  gutters  are  quite  commonly  used  on  streets 
paved  with  macadam,  and  sometimes  at  the  sides 
of  asphalt  streets.  These  are  sometimes  flat  curved 
gutters,  similar  in  form  to  the  cobble  gutter  shown  in 
Fig.  33,  but  more  commonly  they  are  used  with  con- 
crete curbs  as  combined  curb  and  gutter.  These 
consist  simply  of  the  concrete  curb  with  a  concrete 
gutter  1 8  to  30  inches  wide  attached  and  built  together 
in  one  piece.  This  is  usually  placed,  like  a  sidewalk, 
upon  a  layer  of  cinders  or  gravel,  and  is  constructed  in 
the  same  manner,  the  gutter,  upon  streets  of  moderate 
traffic,  being  usually  about  5  or  6  inches  thick. 

Where  the  street  pavement  is  carefully  laid  flush 
with  the  gutter,  this  makes  a  very  satisfactory  gutter. 
Upon  macadam  streets  considerable  trouble  is  some- 
times experienced  in  keeping  the  surf  ace  of  the  macadam 
up  to  the  level  of  the  gutter,  and  in  many  instances, 
unless  considerable  care  is  taken  both  in  construction 
and  maintenance,  a  second  gutter  forms  in  the  surface 
of  the  macadam  next  to  the  concrete  gutter.  This  is 
due  to  the  difficulty  of  properly  compacting  the  mac- 
adam next  to  the  gutter,  as  well  as  to  the  greater  wear 


384       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

of  the  macadam  and  the  effect  of  rain  in  washing  it. 
To  secure  good  results  it  is  essential  that  the  macadam 
be  brought  flush  with  the  concrete  gutter  so  that  the 
water  may  readily  pass  into  the  gutter,  and  that  it  be 
well  compacted  so  as  to  prevent  subsequent  settlement. 

ART.  99.    CROSSINGS. 

On  streets  paved  with  a  smooth  hard  surface  which 
is  easily  cleaned,  such  as  brick  or  asphalt,  special  foot- 
way crossings  are  not  usually  required  or  desirable, 
unless  the  foot  travel  be  very  considerable.  On  other 
pavements,  however,  which  are  apt  to  be  rough  to 
walk  upon  or  muddy  in  bad  weather,  as  upon  stone, 
or  macadam,  footways  of  flagstones,  brick,  or  concrete 
are  commonly  provided. 

Stone  crossings  consist  of  flagstones  about  10  or  12 
inches  wide  laid  in  rows  across  the  street,  the  rows 
being  6  or  8  inches  apart  and  paved  between  with  stone 
blocks  set  in  the  ordinary  manner.  The  crossing- 
stones  are  3  or  4  feet  long,  and  at  least  6  inches  thick 
in  order  that  they  may  not  be  broken  by  the  traffic. 
They  should  be  laid  with  close  joints  and  firmly  bedded 
upon  the  foundation. 

Brick  crossings  are  usually  constructed  in  the  same 
manner  as  brick  street  pavements,  being  laid  upon 
concrete  base  and  having  joints  filled  with  cement 
mortar.  They  should  be  slightly  crowned,  so  as  to 
raise  the  crossing  a  little  above  the  general  level  of  the 
street  and  facilitate  keeping  them  clean.  These  cross- 
ings are  sometimes  laid  as  double  layer  brick  pavements 
with  sand  filled  joints,  but  in  general  the  better  grade 
of  construction  is  but  slightly  more  expensive  and  is 
much  more  durable  in  use. 


CITY  STREETS.  385 

Concrete  street  crossings  are  placed  in  the  same 
manner  as  concrete  sidewalks.  They  should,  like  brick 
crossings,  have  a  crown  to  aid  in  keeping  them  cteaii. 
Crossings,  and  sidewalks  across  alley  openings  or  drive- 
ways, need  to  be  somewhat  heavier  than  ordinary 
sidewalks  and  are  usually  about  6  inches  thick.  It  is 
common  to  cut  longitudinal  V-shaped  grooves,  about 
\  inch  to  I  inch  wide,  i  to  ^  inch  deep,  and  4  inches 
apart,  in  the  surface  of  the  walk  to  afford  a  foothold  to 
horses  in  crossing  it.  These  grooves  may  readily  be 
formed  by  use  of  the  tool  used  in  finishing  joints,  and 
are  of  material  benefit  in  preventing  the  slipping  of 
horses. 

At  street  intersections  where  the  number  of  pedes- 
trians is  large  it  is  desirable  that  the  crossing  be  carried 
across  on  the  level  of  the  top  of  the  curb  without 
leaving  a  step  at  the  gutter  crossing.  This  may  be 
accomplished  by  bridging  over  the  gutter  with  a  flag- 
stone or  iron  plate,  or  by  placing  the  outlets  for  surface 
drainage  a  few  feet  back  from  the  corner  and  eliminating 
the  gutter  at  the  corner. 

ART.  100.    STREET-RAILWAY  TRACK. 

Track  for  street  railways  upon  paved  streets  should 
be  constructed  with  a  view  to  offering  as  little  obstruc- 
tion to  ordinary  street  traffic  as  possible,  while  per- 
mitting the  ready  operation  of  the  railway.  These 
two  points  are  apt  to  conflict,  as  the  interest  of  the 
railway  company  in  the  construction  of  track  is  rarely 
identical  with  that  of  the  public  use  of  the  street. 

Any  street-car  track  is  objectionable  on  a  paved 
street,  both  on  account  of  the  increased  wear  caused 
to  the  pavement,  and  because  it  forms  an  obstruction 


386       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

to  the  ordinary  traffic  of  the  street.  It  is,  however, 
a  necessary  evil,  being  required  for  the  convenience 
of  the  public,  and  its  detrimental  effects  may  often  be 
greatly  lessened  by  proper  attention  to  the  methods 
of  construction  employed.  On  smooth  pavements 
properly  constructed  track  should  offer  no  obstruction 
to  vehicles  crossing  it,  and  afford  no  channels  in  which 
the  wheels  of  vehicles  may  run  and  which  prevent 
wheels  readily  leaving  the  track. 

This  requires  that  the  surface  of  the  pavement  be 
flush  with  the  top  of  the  rail,  and  that  it  be  laid  in 
close  contact  with  the  rail. 

It  is  also  important  that  the  method  of  construction 
used  in  both  track  and  pavement  be  firm  and  substan- 
tial to  prevent  unevenness  due  to  the  yielding  of  the 
track  or  settlement  of  the  pavement. 

Construction  of  Track.  Methods  of  construction 
used  for  street  railway  tracks  are  extremely  various 
and  opinions  differ  widely  concerning  them.  When 
the  traffic  of  the  railway  and  street  is  light  it  is  gen- 
erally conceded  that  the  most  economical  method  is 
that  of  placing  the  rails  directly  upon  wooden  cross- 
ties,  as  in  the  construction  of  steam  roads.  Where, 
however,  the  traffic  is  heavy  the  difficulty  and  ex- 
pense of  making  repairs  becomes  great,  and  the  rail- 
way companies  commonly  recognize  the  advantage  of 
solid  and  permanent  construction.  Several  methods 
have  therefore  been  devised  for  securing  firm  support 
to  the  rails. 

Fig.  41  shows  the  ordinary  method  of  construction 
where  a  concrete  base  is  employed  for  the  pavement 
and  the  tie  is  embedded  in  the  concrete.  In  this  con- 
struction the  track  is  surfaced  up  by  ballasting  in  the 
usual  manner  under  the  ties  with  gravel  or  broken 


CITY  STREETS. 


387 


stone,  after  which  the  concrete  base  is  filled  in  between 
and  perhaps  over  the  ties.  The  depth  of  rail  is  some- 
times made  the  same  as  the  thickness  of  the  upper 
layers  of  pavement,  thus  bringing  the  top  of  the  tie 


FIG.  41. 

even  with  the  surface  of  the  concrete.  Thus  a  six- 
inch  rail  may  be  used  with  a  brick  pavement  having 
a  two-inch  sand  cushion  as  shown  in  Fig.  42.  If  the 
depth  of  rail  be  less  than  this,  stringers,  as  in  Fig.  43, 


FIG.  42. 

or  chairs,  as  in  Fig.  44,  are  necessary  to  raise  the  rails 
to  the  level  of  the  paving  surface.  When  stringers 
are  employed  they  are  usually  connected  by  cast-iron 
braces  to  the  cross-ties,  and  are  also  bedded  and  held 
in  place  by  the  concrete  base  of  the  pavement.  The 
ties  in  such  case  are  usually  below  the  concrete.  To 
secure  greater  stability  when  the  rails  are  supported 


388       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

by  cross-ties  a  bed  of  concrete  is  sometimes  placed 
under  each  tie  and  the  track  is  tamped  in  concrete. 
This  is  shown  in  Figs.  42  and  44.  In  such  construc- 
tion it  is  usual  to  make  a  trench  under  the  tie,  fill  this 


FIG.  43. 

with  concrete  and  tamp  the  tie  with  concrete,  after- 
ward placing  the  concrete  base  for  the  pavement  be- 
tween the  ties. 

On  important  lines  in  streets  of  heavy  traffic  re- 
pairs to  track  are  often  both  difficult  and  expensive, 


FIG.  44- 

and  very  rigid  and  substantial  construction  is  essential 
to  an  economical  operation  of  the  railway.  To  secure 
such  construction  the  wooden  ties  are  sometimes  dis- 
pensed with  and  the  rails  placed  directly  upon  the 
concrete.  Several  methods  of  construction  of  this 
character  have  been  employed.  Fig.  45  shows  the 
simplest  form,  where  the  rails  are  placed  directly  upon 


CITY  STREETS. 


389 


the  concrete  base  of  the  pavement  and  spaced  by  iron 
tie-rods  at  intervals  of  six  or  eight  feet.  The  con- 
crete in  this  construction  is  usually  made  extra  heavy 
in  order  to  adequately  support  the  rails  and  maintain 
them  at  the  level  of  the  pavement. 

A  more  economical  method  of  securing  permanent 


FiG.~45- 

construction  is  by  the  use  of  a  concrete  beam  or 
stringer  under  the  rail.  This  method  of  construction 
is  illustrated  in  Fig.  46.  The  concrete  stringers  are 
usually  made  of  Portland-cement  concrete  from  9  to 


FIG.  46. 

12  inches  in  depth  and  12  to  1 8  inches  in  width  The 
rails  are  commonly  held  in  place  by  iron  ties,  to  which 
the  base  of  the  rail  is  bolted,  as  shown  in  figure.  Fre- 
quently light  angle-bars  are  used  for  ties,  but  various 
other  sections  have  also  been  employed  for  which 
special  advantages  are  claimed.  Spacing  rods  between 
the  webs  of  the  rails  may  also  be  employed  as  shown 
in  Fig.  45,  but  these  rods  are  objectionable  in  a  block 
pavement  on  account  of  the  difficulty  of  paving  be- 


3QO       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

tween  them.  The  ties  are  usually  spaced  about  ten 
feet  apart. 

In  some  instances,  where  concrete  beams  are  used 
under  the  rails,  ties  are  omitted  altogether  and  the 
base  of  the  rail  is  spiked  directly  to  the  concrete  or 
bolted  through  the  concrete  beam  to  plates  below. 
This  latter  method  has  been  carried  out  at  Rochester, 
N.  Y.,  with  entire  success,  no  difficulty  being  experi- 
enced in  holding  the  rails  in  position.  In  construct- 
ing such  track  the  rails  are  usually  laid  on  temporary 
wooden  ties  spaced  ten  or  twelve  feet  apart  and 
brought  to  line  and  grade,  after  which  the  concrete 
beams  are  placed  and  the  wooden  ties  removed. 

Form  for  Rails.  The  rails  in  common  use  for 
street-railway  track  are  divided  into  two  general 
classes:  tee  rails,  as  commonly  used  on  steam  roads, 
and  girder  rails,  in  which  the  head  is  so  formed  as  to 
afford  a  channel  for  the  flanges  of  the  wheels  and  ad- 
mit of  the  pavement  being  laid  close  against  the  rail 
on  both  sides. 

Tee  rails  differ  considerably  in  their  details  and 
weights  and  are  often  modified  for  street  service  by 
making  them  of  greater  depth  than  is  usual  for  steam- 
road  service.  These  rails  are  shown  in  Figs.  42  and 
43.  The  upper  surface  varies  from  2  to  3  inches  in 
width  and  is  usually  made  convex,  the  section  being 
frequently  circular,  of  radius  8  to  20  inches.  As  used 
for  street  railways,  these  rails  vary  from  4  to  7  inches 
in  height.  In  using  the  smaller  depths  it  is  necessary, 
except  for  very  thin  paving  surfaces,  that  the  rails  be 
supported  on  chairs  or  stringers  to  give  room  for  pav- 
ing over  the  cross-ties,  and  deeper  sections  are  there- 
fore more  commonly  used.  The  six-inch  depth  is  fre- 
quently employed  and  is  sufficient  with  an  asphalt  or 


CITY  STREETS.  391 

brick  pavement,  the  ties,  if  used,  being  embedded  in 
the  concrete  foundation. 

The  disadvantage  of  the  tee  rail  consists  in  the  fact 
that  the  pavement  cannot  come  against  the  rail  on  the 
inside  of  the  track,  as  it  has  no  groove  for  the  wheel- 
flange.  The  pavement  must  therefore  either  be  low- 
ered under  the  rail-flange  or  a  groove  be  left  between 
the  head  of  the  rail  and  the  pavement.  The  first 
method,  shown  in  Fig.  42,  is  ordinarily  the  best  con- 
struction, as  the  pavement  is  set  firmly  against  the  rails 
and  there  are  no  exposed  edges  to  cause  rapid  wear,  but 
it  is  objectionable  on  account  of  the  impact  of  wheels 
crossing  in  dropping  from  the  rail,  and  because  it  tends 
to  hold  the  wheels  of  vehicles  in  the  track.  The 
second  method  is  accomplished  by  using  a  thin  block 
or  a  filling  of  concrete  under  the  head  of  the  rail  and 
paving  against  this  filling,  as  is  usual  when  stone-block 
pavement  is  employed  between  the  rails,  or  when  a 
toothing  of  stone  blocks  or  bricks  is  employed  with 
an  asphalt  pavement.  For  brick  pavements  special 
bricks  are  sometimes  molded  to  fit  against  the  rails, 
leaving  a  groove  for  the  wheel-flanges,  as  shown  in 
Fig.  43.  Difficulty  has  sometimes  been  met  in  the 
use  of  these  bricks  on  account  of  their  tendency  to  tilt 
when  the  car-wheel  flanges  press  down  any  dirt  or 
small  gravel  which  may  fall  into  the  groove,  unless 
they  are  very  firmly  bedded  next  the  rail.  Tee-rail 
construction  is  very  commonly  preferred  by  railway 
companies,  as  giving  a  better  road  for  operation.  It 
affords  cheap  construction,  has  little  tendency  to  be- 
come clogged  with  dirt,  and  will  usually  be  avoided 
by  the  ordinary  traffic  of  the  street,  not  affording  good 
channels  for  the  wheels  of  vehicles. 

Girder  rails  are   divided  as  to  form  of  head  into 


392        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 


center-bearing,  side-bearing,  and  grooved.  They  vary, 
as  commonly  used,  from  6  to  9  inches  in  height,  and 
each  type  is  subject  to  several  variations  in  form. 

The  center-bearing  rail  is  shown  in  Fig.  47.  It  is 
the  most  objectionable  of  any  of  the  forms  in  use, 
there  being  two  channels,  one  on  each  side  of  the 
head,  thus  offering  a  double  obstruction 
to  traffic  and  causing  greatly  increased 
wear  to  the  pavement.  It  is  of  advan- 
tage to  the  traffic  of  the  railway  because 
it  does  not  retain  dirt,  and  where  streets 
are  not  kept  in  good  condition  cleanses 
itself,  which  is  particularly  important 
on  electric  roads  in  which  the  rail  is  used 
as  a  current  conductor.  Its  objectionable  features, 
however,  prevent  its  use  in  most  places. 

Side-bearing  rails  are  shown  in  Figs.  44,  48,  and  49. 
They  are   probably  more   commonly  used  than   any 


FIG.  47. 


FIG.  48. 

other  type  of  girder  rail.  The  tram  is  from  2  to  3 
inches  wide  and  offers  a  smooth  track  for  the  wheels 
of  vehicles,  but  it  is  difficult  for  a  wheel  to  leave  it 
and  is  extremely  hard  on  the  street  traffic. 

Pavements  may  be  laid  against  the  side-bearing  rail 
as  shown  in  Fig.  44,  in  which  the  surface  of  the  pave; 
ment  is  at  the  same  level  inside  as  outside  the  track; 


CITY  STREETS.  393 

or,  as  shown  in  Fig.  48,  in  which  the  pavement  inside 
the  track  is  brought  even  with  the  top  of  the  tram  of 
the  rail.  The  first  method  leaves  an  exposed  edge 
of  the  paving  surface,  which  is  commonly  subject  to 
rapid  wear,  wrhile  the  width  of  tram  is  sufficient  to  per- 
mit the  wheels  of  vehicles  to  run  in  the  grooves  and  to 
leave  the  track  with  difficulty. 

Grooved  rails  are  shown  in  Figs.  41,  45,  46,  50,  and 
51.  There  are  many  variations  in  the  form  of  groove 
and  lip  designed  to  meet  varying  conditions  of  use. 
The  full-groove  rail,  shown  in  Fig.  41,  has  a  groove  in 
the  head  usually  from  an  inch  to  an  inch  and  a  quarter 
in  width;  and  when  the  pavement  is  made  flush  with 
the  top  of  the  rail  it  presents  no  obstruction  to  traffic 
of  the  street,  and  as  the  groove  is  too  narrow  to  admit 
the  wheels  of  vehicles  it  forms  the  most  desirable  track 
for  use  with  smooth  pavements.  It  can  only,  however, 
be  used  where  pavements  are  kept  clean  and  in  good 
condition,  as  the  groove  is  otherwise  easity  clogged  with 
dirt,  rendering  the  operation  of  the  railway  difficult 
and  expensive.  This  disadvantage  is  greater  in  cold 
climates  where  snow  and  ice  are  common  during 
winter. 

For  the  purpose  of  lessening  the  clogging  of  the 
groove,  the  form  of  the  grooved  rail  is  sometimes 
modified  by  sloping  the  lip  and  widening  the  groove 
at  the  top,  as  shown  in  Fig.  46  or,  as  shown  in  Fig.  50, 
by  making  the  lip  of  a  less  height  than  the  head  of  the 
rail,  thus  allowing  the  wheel-flange  to  clear  the  groove 
of  dirt  in  passing.  This  latter  form,  however,  has 
the  effect  of  forming  a  track  which  retains  the  wheels  of 
vehicles,  as  will  any  difference  of  elevation  between  the 
head  of  the  rail  and  the  pavement  between  the  rails. 

In  Fig.  51  is  shown  a  grooved  rail  with  an  exten- 


394        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

sion  of  the  lip  intended  to  form  a  track  for  wheels  of 
vehicles  with  a  view  to  reducing  the  wear  of  the  pave- 
ment which  commonly  takes  place  immediately  inside 
the  rails.  When  this  lip  is  below  the  level  of  the  rail- 
head it  is  subject  to  the  same  objection  as  the  side- 
bearing  rail  of  forming  a  track  which  it  is  difficult 
for  wheels  to  leave. 

For  paved  streets,  where  the  pavement  is  well  kept, 
the  grooved  rail  seems  to  be  superior  to  any  other, 
and  is  often  required  by  municipal  authorities,  par- 


FIG.  49. 


FIG.  50. 


FIG.  51. 


ticularly  in  the  larger  cities.  For  unimproved  streets 
or  on  macadam  or  earth  roads  the  tee  rail  is  usually 
considered  preferable,  and  may  usually  be  employed 
with  no  more  injury  to  the  street  traffic  than  any  of 
the  others,  while  possessing  the  advantage  of  economy 
both  in  cost  and  operation  to  the  railway. 

Joints  and  Fastenings.  The  solid  construction  of 
track  is  a  matter  of  importance  upon  paved  streets, 
because  of  the  difficulty  and  expense  of  getting  at  the 
track  to  make  repairs,  as  well  as  because  of  the  dis^ 
turbance  to  traffic  when  the  pavement  must  be  removed 


CITY  STREETS.  395 

for  this  purpose.  The  rail-joints  and  tie-connections 
are  therefore  matters  requiring  particular  attention. 
Where  no  chairs  are  used,  the  use  of  tie-plates  to  form 
a  bearing  for  the  rail  upon  the  tie,  and  to  hold  it 
securely  in  place,  is  to  be  recommended,  and  will 
greatly  aid  in  forming  a  rigid  track.  There  are  a 
number  of  forms  in  use  which  give  good  results.  They 
should  be  arranged  to  clamp  the  rail  firmly  and  present 
a  good  bearing  upon  the  tie.  When  chairs  are  used, 
they,  like  the  tie-plates,  should  clamp  the  rail  firmly 
and  give  good  bearing  surface.  They  should  also  be 
well  braced  for  stiffness  against  lateral  bending. 

Joints,  in  the  case  of  track  formed  of  rails  laid  directly 
upon  the  ties,  or  upon  wooden  stringers,  are  usually 
made  by  placing  a  plate  or  channel-bar  upon  each  side 
of  the  web  of  the  rail-ends  to  be  joined  and  bolting 
through.  The  use  of  slightly  curved  channel-bars 
fitting  against  the  flanges  of  the  rail,  as  shown 
in  Figs.  49  and  51,  seems  to  give  good  results,  the 
spring  in  the  channels  serving  to  prevent  the  loosening 
of  the  bolts.  This  is  the  most  common  method  of 
making  joints.  Fig.  50  shows  a  pair  of  ribbed-joint 
plates  as  used  for  high  rails,  the  center  bearing  serving 
to  prevent  the  buckling  of  the  plates  or  the  bending 
of  the  rail  at  the  ends. 

For  track  in  pavements  the  rails  may  be  laid  to 
close  joints,  no  allowance  being  necessary  for  change 
of  temperature  when  the  rail  is  fully  bedded  in  the 
pavement. 

A  number  of  modifications  of  the  above  joints  have 
been  devised,  some  of  them  passing  under  the  base  of 
the  rail  and  supporting  it  on  the  tie.  Electrically 
welded  or  cast  joints  are  also  sometimes  employed, 
consisting  in  welding  a  bar  of  steel  on  each  side  of  the 


396        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

end  of  the  rail,  or  in  casting  an  iron  block  about  the 
ends  to  be  joined,  the  casting  being  joined  by  means 
of  the  holes  through  the  web  of  the  rails. 

Where  chairs  are  employed  to  raise  the  rails  above 
the  ties,  joints  are  frequently  most  satisfactorily  made 
upon  long  chairs  or  bridges  reaching  across  the  space 
between  two  ties  and  forming  a  firm  bearing  for  the 
ends  of  the  rails. 

In  order  to  facilitate  keeping  the  joints  tight  and 
enable  the  bolts  at  the  rail-ends  to  be  screwed  up 
without  taking  up  the  pavement,  joint-boxes  are 
sometimes  employed.  These  consist  of  openings  with 
removable  covers,  giving  access  to  the  bolts  at  the 
ends  of  the  rails . 

On  curves,  guard-rails  are  commonly  employed. 
Where  tee  rails  are  employed  the  guard  is  usually  a 
second  rail  placed  on  the  inside  of  the  main  rail,  leaving 
only  room  for  the  wheel-flanges.  In  some  instances, 
however,  the  guard  is  formed  by  bolting  a  flange  to 
the  main  rail.  For  girder  rails  the  guard  is  usually 
formed  by  the  use  of  a  rail  in  which  the  groove  is 
wider  and  the  lip  heavier  than  common,  and  sometimes 
the  lip  extends  somewhat  above  the  head  of  the  rail. 
Any  difference  of  elevation  of  that  kind  is  objection- 
able as  producing  unevenness  in  the  pavement,  but  is 
frequently  used  as  essential  to  the-  proper  operation  of 
the  cars  upon  the  curves. 

Pavement  in  Car  Tracks.  The  wear  of  a  pavement 
is  usually  considerably  increased  by  railway  tracks 
upon  the  street.  The  extent  of  this  wear  depends 
upon  the'  nature  of  the  paving  surface  as  well  as  upon 
the  construction  of  the  track.  It  is  mainly  the  differ- 
ence in  resistance  to  abrasive  wear  between  the  rails 
and  the  paving  surface  which  causes  uneven  and  more 


CITY   STREETS.  397 

rapid  wear  of  the  pavement  in  vicinity  of  the  track, 
A  broken-stone  surface,  on  account  of  its  rapid  wear, 
is  particularly  objectionable  along  a  line  of  track,  and 
is  very  difficult  to  keep  in  proper  surface. 

In  case  of  narrow  streets  or  rough  side-pave- 
ments the  use  of  the  track  for  hauling  heavy  loads 
causes  the  cutting  of  the  pavement  upon  the  outside 
of  the  track,  due  to  the  gauge  of  trucks  being  greater 
than  that  of  the  track.  This  is  especially  the  case 
where,  owing  to  the  use  of  side-bearing  or  center- 
bearing  rails,  the  flange-grooves  are  wide  enough  to 
permit  the  wheels  of  trucks  to  enter  them. 

Where  tracks  follow  county  roads  it  is  usualh7 
desirable,  if  possible,  to  place  the  track  at  one  side 
and  leave  the  center  of  the  street  free  for  the  use  of 
the  ordinary  traffic.  When  a  broken-stone  or  gravel 
surface  is  emplo\Ted  it  is  common  to  lay  planks  on 
each  side  of  the  rail  and  bring  the  pavement  against 
the  planks,  which  materially  lessens  the  obstruction 
offered  to  travel  by  the  rails,  as  well  as  the  difficulty 
of  keeping  the  pavement  in  surface. 

The  methods  of  placing  pavements  in  tracks 
depend  upon  the  shape  of  the  rail-heads  and  have 
already  been  discussed.  Under  heavy  traffic  when 
asphalt  street  surface  is  employed  it  is  quite  common 
to  pave  between  the  rails  with  stone  or  brick,  and 
often  to  put  a  toothing  of  the  same  material  outside 
the  rails  adjoining  the  asphalt.  This  serves  to  prevent 
the  cutting  of  the  asphalt  along  the  rails.  Sometimes 
when  stone  blocks  are  used  in  track  the  concrete  base 
is  omitted  and  the  blocks  are  set  on  a  gravel  or  broken- 
stone  base.  When  such  construction  is  emplo\Ted  the 
track  should  be  ver\r  carefully  ballasted  and  brought 
to  an  even  bearing.  There  is  always  a  tendency  for 


398       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

the  track  to  work  loose  from  the  pavement  and  get 
out  of  surface,  and  under  heavy  traffic  very  firm  con- 
struction is  necessary  to  counteract  this  tendency. 

ART.  101.    TREES  FOR  STREETS. 

It  is  always  desirable,  wherever  possible,  to  have 
streets,  at  least  those  devoted  to  residential  purposes, 
lined  with  rows  of  trees  upon  each  side,  both  for  the 
purpose  of  giving  shade  and  to  add  to  the  beauty  of 
appearance  of  the  street. 

The  most  satisfactory  way  of  arranging  trees  is  usually 
to  have  a  tree  space  between  the  sidewalk  and  the  curb 
in  which  the  trees  are  planted  in  a  straight  line  along 
the  street.  Sometimes  in  ver}^  wide  streets  a  tree 
space  or  parking  is  arranged  in  the  middle  of  the  street 
with  a  driveway  on  each  side.  Trees  should  be  spaced 
in  the  rows  at  such  distances  as  will  permit  each  tree 
when  fully  grown  to  spread  to  its  full  natural  dimen- 
sions, which  usually  requires,  for  trees  ordinarily  em- 
ployed, from  25  to  40  feet. 

The  selection  of  the  variety  of  trees  to  be  used  for 
this  purpose  must  of  course  depend  upon  climatic 
and  local  conditions.  Those  which  rapidly  attain  their 
full  size  are  usuall}"  to  be  preferred.  They  should 
have  a  graceful  form  and  make  a  good  shade,  but  the 
foliage  should  not  be  too  dense.  Evergreens  are  not 
generally  desirable  for  this  purpose.  Where  there  is 
plenty  of  room  for  their  development  the  large-grow- 
ing varieties  with  light  foliage  are  handsome  and  desir- 
able. The  size,  however,  must  be  suited  to  the  space, 
and  upon  narrow  streets,  or  where  the  trees  are  to  be 
close  to  the  buildings,  they  must  be  of  small  growth. 
The  ease  with  which  the  tree  may  be  grown  and  its 


CITY  STREETS.  399 

liability  to  disease  or  to  be  affected  by  the  contamina- 
tions of  a  city  atmosphere  must  be  considered,  as  the 
conditions  under  which  street  trees  must  be  grown  are 
not  usually  favorable  to  their  best  development. 

It  is  desirable,  especially  in  cities  of  considerable 
size,  that  the  planting  and  care  of  trees  be  under  con- 
trol of  the  municipal  authorities.  Trees  may  then  be 
set  with  a  view  to  the  best  general  effect  upon  the 
street  as  a  whole;  the  selection  and  planting  of  the 
trees  ma}^  be  property  done,  and  the  trees  after  plant- 
ing may  be  systematically  cared  for. 

ART.  102.    SELECTION  OF  PAVEMENTS. 

The  selection  of  the  best  pavement  for  use  in  any 
given  instance  involves  a  study  of  the  characteristics 
of  each  material  as  to  its  fitness  for  the  particular 
service  required,  its  suitability  for  meeting  the  local 
conditions  under  which  it  is  to  be  used,  and  its  probable 
cost.  Local  conditions  must  be  taken  into  account, 
and  it  is  not  possible  to  lay  down  any  fixed  rules  for 
universal  application.  The  availability  of  materials 
in  the  locality,  and  relative  costs  which  vary  with  local 
conditions,  are  frequently  determining  factors  in  the 
choice  of  pavements. 

A  good  pavement  should  present  a  smooth,  hard,  and 
impervious  surface,  which  may  be  easily  cleaned  and 
offers  small  resistance  to  traction.  The  comfort,  con- 
venience, and  health  of  people  using  the  street  and  of 
residents  of  the  locality  may  be  largely  affected  by  the 
character  of  a  street,  and  should  be  the  first  considera- 
tion in  deciding  upon  an  improvement. 


400        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 
HEALTHFULNESS. 

The  effect  of  a  pavement  upon  the  health  of  the 
residents  of  its  locality  will  be  affected  by  the  tendency 
of  the  materials  composing  it  to  decay,  by  its  permea- 
bility, and  by  its  degree  of  freedom  from  noise  and 
dust. 

The  permeability  of  a  road  surface  is  important  on 
account  of  the  tendency  of  surface  water  and  refuse 
matter  to  penetrate  and  saturate  it,  and  thus  cause  it 
to  become  dangerous  to  health.  A  continuous  sheet 
pavement  is  the  most  desirable  in  this  particular,  and 
a  block  pavement  with  open  joints  the  least  so.  When, 
however,  the  joints  of  a  block  pavement  are  properly 
cemented,  the  pavement  may  be  made  nearly  imper- 
vious, and  any  of  the  pavements  in  common  use,  when 
well  constructed,  are  practically  impervious  to  water. 

Noiselessness.  The  noise  made  by  traffic  upon  a 
pavement  is  important  not  only  because  of  its  effect 
upon  the  comfort  of  the  people  using  it  or  living 
adjacent  to  it,  but  also  because  to  it  are  frequently 
attributed  many  nervous  disorders  to  which  people 
in  some  cities  are  subject. 

Stone-block  pavements  are  the  most  objectionable 
in  this  particular,  causing  a  continual  roar,  due  both 
to  the  rumbling  of  wheels  over  them  and  the  blows  of 
the  horses'  feet  upon  them.  Upon  asphalt  the  noise  is 
only  that  due  to  the  horses'  feet,  giving  a  sharp, 
clicking  sound.  Upon  wood  the  horses  produce  no 
appreciable  sound;  but  wheels  give  a  dull  rumble, 
generally  considered  the  least  objectionable  of  any  of 
the  noises  made  by  the  more  common  pavements. 
The  noise  of  wood  pavements  is  diminished  by  mak- 
ing the  joints  between  blocks  small,  and  a  well  con- 


CITY  STREETS.  40 1 

structed  wood-block  pavement  is  usually  the  least 
noisy  of  the  pavements  in  common  use.  The  noise 
made  by  traffic  upon  a  brick  pavement  varies  with 
the  method  of  construction.  The  clicking  sound 
made  by  horses  is  less  than  on  asphalt,  but  the  rumble 
of  vehicles  is  greater,  the  rumble  being  usually  more 
objectionable  with  hydraulic  cement  than  with  bitu- 
minous or  sand-filled  joints,  although  when  proper 
expansion  joints  are  used  with  cement  joints  the  noise 
is  not  ex(  essive. 

Broken -stone  roads  are  less  nois}^  than  any  of  the 
harder  pavements  excepting  wood  blocks,  while  earth 
roads  are  the  most  desirable  on  this  account  when  in 
smooth  condition. 

Freedom  from  Dust.  The  dust  arising  from  a  pave- 
ment is  objectionable  on  the  score  of  health  as  well 
as  of  comfort.  For  the  most  part  the  dust  found  upon 
city  pavements  is  produced  from  dirt  carried  there 
from  the  outside.  To  eliminate  this  it  is  necessary  to 
keep  the  pavement  clean,  and  perhaps  to  sprinkle  it. 
All  pavements  produce  more  or  less  dust,  even  when 
kept  thoroughly  cleaned.  Stone,  brick,  and  asphalt 
surfaces  all  give  off  a  small  amount  of  very  fine  dust, 
which  rises  in  the  wind  unless  the  surface  is  kept 
sprinkled.  Wood-block  surfaces  are  less  objectionable 
on  this  account.  Broken -stone  roads  wear  rapidly 
and  make  dust  freely  in  dry  weather,  unless  kept 
sprinkled  or  treated  with  oil  or  tar  (see  Art.  41). 

SAFETY. 

The  safety  of  a  road  surface  depends  upon  the  foot- 
hold afforded  by  it  to  horses  under  normal  conditions, 
and  also  upon  the  degree  of  slipperiness  that  it  may 


402       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 


take  in  wet  weather,  or  under  the  influence  of  ice  and 
snow  in  winter. 

A  dry  earth  road  in  good  condition  gives  the  best 
and  surest  foothold,  with  broken-stone  and  gravel 
roads  nearly  as  good. 

The  relative  safety  of  the  various  pavements  used  in 
city  streets  is  a  matter  upon  which  there  is  consider- 
able difference  of  opinion  amongst  authorities.  Local 
conditions  affect  the  pavement  in  this  regard  to  an  im- 
portant degree.  The  dampness  of  the  climate,  the 
shade  from  buildings,  the  cleanliness  of  the  streets, 
and  the  prevalence  of  snow  and  ice  in  winter  are  all 
important. 

Statistics  upon  the  question  of  relative  safety  of 
wood,  asphalt,  and  granite  have  been  collected  by 
Captain  Greene  in  this  country  and  by  Colonel 
Haywood  in  London,  the  attempt  being  made  to  deter- 
mine the  number  of  miles  traveled  by  horses  upon  each 
kind  of  pavement  to  each  accident  due  to  slipperiness. 

The  results  of  Colonel  Haywood  seem  to  show  that 
of  the  three,  wood  is  the  safest  and  granite  the  most 
dangerous,  while  the  results  of  Captain  Greene  show 
asphalt  to  be  the  best  and  wood  the  worst  in  this 
particular. 

Colonel  Haywood's  observations  were  all  taken  on 
London  streets,  and  are  as  follows: 


Miles  Traveled  to  Each  Fall  on  — 

Granite. 

Asphalt. 

Wood. 

In  dry  weather 

78 
168 
432 
132 

223 
125 
192 
191 

646 
193 
537 
33° 

In  damp  weather 

In  wet  weather  

All  observations  

CITY  STREETS.  403 

The  observations  were  made  when  dry  weather 
prevailed,  and  therefore  are  somewhat  unfavorable  to 
granite,  which  is  safest  when  wet. 

Captain  Greene's  observations  were  made  in  several 
American  cities,  and  showed  the  distance  traveled  to 
each  fall  to  be,  on  granite  413  miles,  on  asphalt  583 
miles,  and  on  wood  272  miles.  The  observations  on 
wood  in  this  series  were  too  few  to  give  a  reliable  in- 
dication, and  it  is  to  be  observed  with  regard  to  all  of 
them  that  slipperiness  is  largely  affected  by  the  con- 
dition in  which  the  surface  is  maintained,  and  it  is 
therefore  difficult  to  draw  any  general  conclusions 
which  would  fit  all  cases. 

All  hard  pavements  are  slippery  when  muddy  and 
wet,  and  cleanliness  is  the  necessary  condition  of 
safety. 

Wood  and  asphalt,  if  clean,  are  least  slippery  when 
dry  and  most  so  when  simply  damp.  Granite,  after 
the  surface  becomes  worn  and  polished,  is  most  slip- 
pery when  dry  and  least  so  when  wet. 

Under  a  light  fall  of  snow  both  wood  and  asphalt 
become  very  slippery,  and  in  freezing  weather  wood 
sometimes  becomes  slippery  through  the  freezing  of 
the  moisture  retained  by  it. 

No  statistics  are  available  as  to  the  safety  of  brick 
pavements,  but  it  is  thought  a  desirable  material  in 
this  respect. 

It  may  also  be  remarked  that  the  danger  of  a  horse 
falling  upon  any  pavement  depends  very  largely  upon 
the  training  of  the  animal  and  whether  he  be  accus- 
tomed to  the  particular  surface  in  question. 


404        A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 
DURABILITY   OF   VARIOUS   SURFACES. 

The  durability  of  a  road  or  pavement  is  dependent 
upon  so  many  circumstances  connected  with  local  con- 
ditions, the  nature  of  the  traffic,  methods  of  con- 
struction, and  efficiency  of  maintenance,  that  am^ 
comparison  of  the  various  kinds  of  pavement  in  this 
respect  is  difficult  and  likely  to  be  misleading. 

The  qualities  which  especially  affect  the  durability 
of  the  road  may  be  partially  enumerated  as  follows: 

(1)  The  hardness  and  toughness  of  the  material  com- 
posing the  surface,  upon  which  depends  the  resistance 
of  the  surface  to  the  abrading  action  of  the  wheels  and 
horses'  feet  passing  over  it. 

(2)  The  firmness  of  the  foundation,  which  serves  to 
distribute  the  loads  over  the  road-bed  and  keep  the 
surface  uniform. 

(3)  The  drainage  of  the  road-bed,  which  can  only 
properly  sustain  the  loads  wrhich  come  upon  it  when  it 
is  dry. 

(4)  The  permeability  of  the  surface,  which  should 
form  a  water-tight  covering  to  serve  the  purpose  of 
keeping  the  foundation   and  road-bed   in  a  dry  con- 
dition. 

(5)  The  resistance  of  the  materials  of  the  pavement 
to  the  disintegrating  influences  of  the  atmosphere  and 
to  the  action  of  the  weather. 

The  relative  importance  of  these  various  factors,  in 
any  particular  case,  depends  largely  upon  the  nature 
and  extent  of  the  traffic  which  is  to  pass  over  the 
pavement. 

The  amount  of  traffic  to  which  a  street  is  subjected 
is  usually  estimated  in  terms  of  tons  per  foot  of  width 
of  street,  by  observing  the  number  of  teams  passing  a 


CITY  STREETS.  405 

given  point  during  certain  times,  classifying  them,  and 
assigning  an  average  value  of  load  to  each  class.  The 
wear  of  the  surface  will  naturally  be  somewhat  propor- 
tional to  the  amount  of  traffic.  The  life  of  a  pave- 
ment is,  however,  affected  by  other  conditions,  and 
hence  cannot  always  be  inferred"  from  the  amount  of 
traffic. 

Traffic  may  also  be  classified  according  to  its  nature 
as  heavy  or  light,  depending  upon  the  weight  of  indi- 
vidual loads  which  are  carried.  It  is  the  heavy  loads 
borne  upon  narrow  wheel-tires  that  do  the  greatest 
damage  to  a  pavement,  and  hence  the  nature  rather 
than  the  amount  of  traffic  determines  the  character  of 
pavement  necessary. 

Granite  blocks,  where  a  firm  unyielding  foundation 
is  emplo3^ed,  give  the  hardest  and  most  durable  surface 
of  any  of  the  common  pavements.  This  is  epecially 
the  case  under  very  heavy  loads. 

The  durability  of  wood-block  pavements  under  wear 
varies  widely  for  the  different  types  of  construction. 
The  better  grades  of  treated  wood-block  pavement  seem 
to  have  given  results,  in  some  instances,  second  only 
to  granite  blocks,  and  they  are  being  used  under  some 
of  the  heaviest  traffic  in  the  larger  cities.  The  older 
and  cheaper  types  of  wood  pavement  are  inferior  in 
wearing  qualities  to  brick  or  asphalt. 

Asphalt  and  brick  pavements  when  well  constructed 
are  satisfactory  under  any  but  the  heaviest  traffic. 
The  relative  durability  under  wear  of  brick  and  asphalt 
is  a  matter  of  doubt,  both  materials  being  subject  to 
considerable  variations  in  quality,  and  showing  varying 
results  in  different  localities,  due  both  to  differences  in 
the  quality  of  the  material  and  in  the  methods  of 
construction.  Bitulithic  may  be  classed  with  asphalt 


406       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS 

as  to  durability,  although  it  seems  in  some  instances 
to  have  shown  greater  resistance  to  wear  than  ordinary 
asphalt. 

Broken  stone  wears  rapidly  under  moderately  heavy 
traffic,  and  should  be  employed  only  on  suburban 
streets  or  country  roads  used  mainly  for  light  driving 
or  a  small  amount  of  traffic. 

ART.  103.     SOURCES  OF  REVENUE  FOR  STREET 
IMPROVEMENT. 

Funds  required  for  the  improvement  of  streets  in 
cities  are  commonly  derived  either  from  general  taxes, 
from  assessment  upon  property  in  vicinity  of  the 
improvement,  or  from  a  combination  of  the  two.  In 
some  instances  also  special  taxes  are  levied  upon 
vehicles,  or  upon  business  interests  which  make  large 
use  of  the  streets,  for  the  benefit  of  the  paving 
funds. 

Mr.  J.'  L.  Van  Ornum  has  brought  together*  data 
showing  the  practice  in  fifty  American  cities.  In  a 
few  of  these  the  whole  charge  is  placed  upon  the  general 
taxes.  In  a  few,  the  whole  charge  is  laid  upon  adjoin- 
ing property,  while  in  a  larger  number  the  cost  is  dis- 
tributed between  the  two  in  varying  proportions. 
Some  cities  pay  for  the  intersections  of  streets  from 
general  taxes,  laying  the  whole  cost  in  the  blocks  upon 
property  fronting  the  street.  Some  also  pay  a  share 
(about  20  per  cent  to  40  per  cent)  of  the  cost  between 
intersections  from  general  taxes;  while  in  other  instances 
the  city  pays  a  fixed  percentage  of  the  whole  cost,  the 
remainder  being  assessed  upon  the  property  benefited. 
Some  cities  pay  for  the  grading  of  the  street  from  general 

*  Transactions  American  Society  of  Civil  Engineers,  Vol.  XXXVIIL 


CITY   STREETS.  407 

funds,  while  others  include  the  grading  in  the  cost  of 
paving  and  assess  it  upon  the  property. 

There  has  been  considerable  discussion  on  the  part  of 
municipal  officers  concerning  the  proper  method  of 
apportionment,  and  many  different  opinions  have  been 
advanced  as  to  what  should  be  required  in  fairness  to 
all  of  the  interests  involved.  Some  contend  that  the 
streets  are  for  general  public  use,  and  that  the  people 
of  the  city  as  a  whole  should  pay  for  their  improvement, 
thus  ignoring  the  advantage  that  the  improvement  of 
streets  may  be  to  the  owners  of  abutting  property  in 
the  increase  of  values.  Others  insist  that  it  is  fair  to 
tax  the  whole  of  the  improvement  upon  abutting 
property,  claiming  that  the  main  benefit  is  to  that 
property,  and  that  when  improvement  becomes  general 
each  will  have  paid  his  proper  proportion  of  cost. 
Between  these  two  extremes  are  the  larger  number  who 
recognize  the  interest  of  .both  parties  and  advocate  the 
division  of  cost  between  the  city  as  a  whole  and  the 
abutting  property  in  varying  proportions.  Opinions 
differ  as  to  what  these  proportions  should  be,  and  it  is 
evident  that  the  public  interest  in  some  streets  of  a 
city  is  much  greater  than  in  others.  Some  streets  are 
main  arteries  for  travel,  others  but  ver}T  little  used,  and 
the  relative  values  to  the  general  public  and  to  the 
property  owner  are  very  different  in  the  two  cases. 

It  would  be  quite  impossible  to  devise  any  general 
method  of  apportioning  the  costs  of  street  work  among 
the  various  interests  involved  in  such  a  way  as  to  tax 
each  in  proportion  to  the  benefit  derived  from  the 
improvement.  It  is  generally  recognized  that  the  city 
as  a  whole  is  interested  in  the  improvement  of  its  streets 
and  also  that  property  in  the  immediate  vicinity  of  an 
improvement  is  directly  benefited  thereby.  Therefore 


408       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

it  may  be  considered  fair  and  reasonable  to  tax  either 
or  both  for  the  improvement.  A  division  of  cost  would 
without  doubt  be  the  rriore  equitable  method,  but  the 
feasibility  of  securing  necessary  funds  for  proper 
improvement  by  one  method  or  the  other  must  be  the 
determining  factor  in  selecting  the  method.  In  some 
instances  where  the  funds  derived  from  general  taxation 
are  closely  limited,  the  assessment  of  the  whole  cost  of 
street  improvements  upon  abutting  property  makes 
possible  an  extent  of  improvement  which  would  other- 
wise be  out  of  the  question,  with  manifest  advantage 
to  the  property  taxed. 

In  assessing  the  cost  of  street  improvement  upon 
property  in  the  locality  of  the  work,  the  usual  method 
is  to  apportion  the  cost  upon  abutting  property  in 
proportion  to  the  frontage  upon  the  street  improved. 
Frequently  the  apportionment  is  made  in  proportion 
to  area  on  each  side  of  the  street  within  a  certain  dis- 
tance of  it  (sometimes  half  a  block).  In  a  few  instances 
a  combination  of  the  two  methods  is  used,  by  which  a 
portion  of  the  cost  is  assessed  upon  abutting  property 
in  proportion  to  frontage  and  the  remainder  upon  area 
within  certain  distance.  The  frontage  assessment 
seems  reasonably  fair  and  is  most  commonly  employed 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


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THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
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OVERDUE. 


NOV«27    1934 


LD  21-100m-7,'33 


YB  10943 


26361 G 

X* 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


